History of York County, Pennsylvania : from the earliest period to the present time, divided into general, special, township and borough histories, with a biographical department appended, Part 92

Author: Gibson, John, Editor
Publication date: 1886
Publisher: F.A. Battey Publishing Co., Chicago
Number of Pages: 1104

USA > Pennsylvania > York County > History of York County, Pennsylvania : from the earliest period to the present time, divided into general, special, township and borough histories, with a biographical department appended > Part 92

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The perpendicular thickness of the Huro- nian rocks which constitute the visible parts of this arch has been calculated by me to amount to 14,400 feet or two and seven-tenths miles (or four and three-tenths kilometers) meas- uring from the lowest rocks exposed a short distance above McCall's Ferry to the base of

*In volume CC, p. 334, of the publications of the Second Geo- logical Survey of Pennsylvania, among the specimens collected during the year 1875 are two, of which the provisional field numbers are 424 and 425, marked respectively "Mesozoie lime- stone (marlite?)" and "mar! (?) Mesozoic limestone conglomer- ate," hoth from Welty's farm, Dillsburg. From the second en- try, it would appear that there were two distinct specimens comprised under the same field number. Whether the same be true of the former number also, or whether I was led at first to regard this marl as a small remaining patch of Cretaceous marl like that nf New Jersey, I cannot now say. Nor have I at hand the evidence which induced the belief, subsequently, that this was of Tertiary (Cainozoic) age. That, however, is my present impression. P. F.

+There are, however, good reasons for rejecting such an estimate.

*See Vol. E. p. 241, Publication of the Second Geological Survey of Pennsylvania.

. fSee these sections in Atlas accompanying Vol. CCC, Second Geological Survey of Pennsylvania, by the writer.

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HISTORY OF YORK COUNTY.

the Peach Bottom slates. This arch (or anti - clinal, as it is technically called) is a very important feature in the geology of this part of the State; for, if my conjecture as to its extent be 'well founded, it is in all proba- bility the leading element in the structure of a broad belt of rocks extending from a point at least north of the Schuylkill River, (and not improbably even within the New England States) to and into the State of Alabama.

But whether this carefully considered hypothesis be true or not, there is not the slightest reason for doubting that the rocks of this part of the county form the floor on which all the others in the county were laid down. Another fact in relation to this flat arch or anticlinal remains to be con- sidered, viz. the line along its crown (or along the top of the arch) appears not to have been a horizontal line after the last great earth- crust movements of which we can find evi- dence in this part of the continent had been completed.

As architects, geometers, and geologists would express it, the axis of this arch appears to have sloped upwards, from the west of south to the east of north. As the axis lies along the summit of the bend in the lower bed over which all the other beds are laid down, it follows that it lies in the lowest beds which form the arch; and to say that this axis rises towards the northeast, is to say that, judged from our present surface, the lower (and consequently older) beds of this arch rise nearer to that surface the farther one follows this direction of northeast, and of course these same rocks sink lower beneath the surface the farther one follows the direc- tion of the arch to the southwest. I have elsewhere given reasons for the hypothesis that this "anticlinal" joins and continues the anticlinal of the Buck Ridge * near Conshohocken, a few miles northwest of Phila- delphia on the Schuylkill River, traversing Lancaster and Chester Counties a little south of the Chester Valley. But at Con- shohocken, the anticlinal is represented by Laurentian gneiss, while in Lancaster and York Counties the Huronian schists, which have been torn off by atmospheric denuda- tion at the former Jocality, still remain; and still farther to the southwest it is not unlikely that even more recent sheathings may be found, unless the axis be broken or bent and rise also in this direction. The main fact, which it is my purpose. to emphasize here, is that the same structure of arch evidently af-

fects an enormous thickness of beds, and in all probability is traced in the flexed rock masses of at least two entirely different geological periods: indeed it may possibly be discovered in those of yet others outside of the limits of the field which it is the purpose of the writer to describe.

A somewhat arbitrary division has been made by the writer between the rocks of the Huronian and those of the next following age. The liue which constitutes this divi- sion may be seen passing through the south- ern part of Lower Windsor, the middle of Windsor, the eastern part of Springfield, in- cluding Codorus, and reaching the Maryland line a short distance east of the boundary dividing Manheim from West Manheim Townships. This line does not profess to be, and in all probability is not, an accurate line of demarcation between the two forma- tions. It was adopted as an approximate dividing line between two regions which ex- hibit lithological characteristics diverging from each other in a degree proportional to the distance on either side of it. The same is true of the line which separates these lower rocks from the triangular area in the extreme southeastern corner of the county, in which are found the justly famous Peach Bottom roofing slates. These two lines, which are in the average parallel to each other, are ap- proximate boundaries only between the two regions and that filled by the rocks of the Mc- Call's Ferry, or the Tocquan Creek anticlinal. The rocks of the latter belt are strongly marked crystallized rocks,* i.e., their structure is coarse and the minerals which compose them are large and well crystallized, espec- ially along the central parts of the belt. The rocks of the two bordering regions just men- tioned are more crystalline, i. e., crys- tallized imperfectly, or in much smaller mass- es, besides having other differences in kind. For example, the arch-belt (if I may be per- mitted to express it so) contains larger amounts and larger specimens of Muscovite and potash micas. The rocks are lighter, and not infrequently enough feldspar is found to give them a decidedly gneissic character; and the more so in general terms the farther one gets away from the bordering regions. The rocks in these latter regions, on the other hand, are more and more magnesian, darker in color (usually greenish or yellowish - green) and softer. They contain large quan- tities of chloritic minerals, and are more re- markable for the great number of white quartz dykes which intersect them than those of any other series in the county; or indeed,

*See "Thèses presentées a la Faculté des Sciences de Lille. Université de France, etc., 1882," and "History of Lancaster County, etc., Philadelphia: Everest & Peck. Published 1883" p. 3.

* See note 1 at the end.

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GEOLOGY.

if they be taken in their entirety, in the United States, than any other rock series rep- resented here.

These "arch-rocks" are very generally destitute of valuable minerals, so far as they have been explored in York County, except on the fringe of the South Mountain, where they are very generally in close proximity with a series of iron ore deposits similar to and in fact continuous with those known as the ores of the "Great," or "Cumberland Valley." But though this juxtaposition would tempt one to connect these ores with the rocks just spoken of, and though it is conceded that rocks of this age do often carry iron ores, the strong probability is that the proximity is "accidental;" that is to say, that the ores occur at the foot of the moun- tain, because having been originally embedded (as constituents of minerals) in the rocks which covered these slopes, during the degra- dation and destruction of these latter, they have been disintegrated, carried away from their original place (sometimes not far off), and segregated in the soft and unctuous clays to which these loose beds have been reduced. But it is not improbable that some of these ores may have owed their origin to the same kind of alteration taking place within the mass of the Huronian rocks themselves. So that wherever the loose debris of higher for- mations (and notably of the Hellam quartzite (Potsdam Sandstone) which everywhere abounds on the slope in boulders and blocks) will permit the undoubted Huronian to ap- pear near one of these great iron mines, it is likely to be found that a part of the wealth of the latter consists of a somewhat peculiar ore, unlike the rest, which can be traced to its first resting place within the bosom of the Huronian rocks.

The belt of rocks which represents the Eozoic in York County, lies, as it may be said approximately, between two lines, one fol- lowing Muddy Creek from its mouth in the Susquehanna to its right angled bend and thence through Bryantsville to Constitution; and the other commencing opposite Turkey Hill (in Lancaster County) and passing north- west of Windsor Postoffice, southeast of Dal- lastown and nearly through Glen Rock Post- office. The portion of the South Mountain above referred to as belonging to the same age is small in area within the county limits, and occurring at the end of one chain of Eozoic rocks where they appear to sink beneath the newer limestones and shales, its slopes are gentler, it has been subjected to greater erosion, and is covered for the most part with the debris of more recent formations.

This belt, thus defined, contains no minerals which are yet mined (if we except the iron ores from the category), but the soil formed by the chemical and mechanical action of the atmosphere on its rocks is next in fertility to that of the limestone belt itself. The rocks of the Eozoic belt thus defined are intersected by but few igneous dykes or trap; and this fact taken in connection with the remarkable prevalence of such dykes in the northwestern part of the county, and their frequency throughout the middle belt of limestone and schists, would lead one to conclude either that the seat of the igneous action resided within the beds of the newer rocks, or that the superposition of the latter in some way favored the development of the Plutonic forces which may have forced molten rock for miles through narrow crevices and cracks in the envelope of the globe. Perhaps the explanation may be found in the supposition that the number of such dykes would depend upon the number of fractures in the earth's crust, and that this number would increase with the growing weight due to thickening sediments deposited by water. However this may be (and it does not explain all of the facts connected with the new red sand- stone), the only points where I have observed trap penetrating the rocks of this belt are: First, in a small exposure north of York Fur- nace on the Susquehanna; and second, a short distance east of Black Rock Furnace.

THE BELT OF UPPER EOZOIC.

I have preferred to describe this belt under a separate heading because there are difficul- ties connected with its assignment either to the Eozoic rocks, just considered, or to the Pal- æozoic, which will next be described. These difficulties arise in great part from the lack of outcrops of "rock in place" or bedded rocks, which remain substantially in the position in which these were deposited and hardened. The decomposition which has attacked this intermediate belt has destroyed the indentity of the individual beds and strewn the sur- face with its products, which are mingled with the remains of rocks of much later date. This is not surprising, if we may assume that this belt formed the upper and later por- tions of the great Eozoic series, for we have abundant proof that in contrast to the stabil- ity and repose of the broad flat arch to the southeast, this new region was the hinge on which the first of a number of severe plica- tions of the strata were operated. This bending and twisting unquestionably crum- bled the rocks and left loose material, which was easily moulded by the waters of the

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HISTORY OF YORK COUNTY.

ocean which subsequently covered it to forms which more or less resembled those which had originally characterized it. But after its consolidation with the next succeeding formation, both were together similarly treat- ed, so that in the contorted state in which it was left, it exhibits some features which re- call the lower Eozoic, and others which remind one of the lower Paleozoic of the county. Its precise boundaries, being difficult to as- certain on the ground, cannot be given with precision in the text. It will suffice to say that beginning on the Susquehanna River a short distance south of the southern outcrop of the Prospect limestone, one part of it oc- cupies all the region lying between the north- western boundary of the Eozoic. already given, and the southern and eastern limits of the Hellam quartzite shortly to be described. It is traversed through part of its extent by two large trap dykes, and contains numerous deposits of iron ore which I am disposed to ascribe to segregation from iron minerals in other formations. Some limestone occurs interbedded with these rocks (as at Glen Rock) which may be safely assumed to be of earlier date than the important York limestone, whether or not it be (as seems not improbable) a part of the regular Huronian series.

The most extensive iron ore banks noted in or on the border of this intermediate belt are the Brillhart and Feigley banks, and marked Nos. 11 and 12 on the map.

The Peach Bottom districts, including the famous roofing slates lying to the south of the flat arch, was described by me in Volume CCC, Second Geological Survey of Pennsyl- vania in 1877, where I showed that its position in the series was doubtful and that these rocks might be interpreted to represent the upper Eozoic (below the Potsdam) or the schists im- mediately above the Potsdam, or (by supposing a fault) a formation still higher-the "Mat- inal" of Rogers. Since then fossil algae were furnished to Prof. James Hall from the quar- ries, but he was unable to determine the age of rocks from them with greater precision than to refer them to the second or third of these horizons with a preference to the sec- ond .* The commercial value of these slates will doubtless be treated elsewhere. Photo- graphs of the quarries, and of the manner of working them. will be found in Volume CCC, second Geological Survey of Pennsylvania.t

THE PALEOZOIC ROCKS. HELLAM QUARTZITE. (POTSDAM SANDSTONE).

Prof. H. D. Rogers, in the first geological

survey of Pennsylvania, marked out and described the members of the different forma- tions represented in the State. This forma- tion, which we may consider the base of the Palæozoic, in a sketch like the present, which is intended for the people and not for spec- ialists, was considered by him to consist of three parts: a lower series of "talcose" slates, a middle white sandstone and an upper series of talcose slates. It will be easily understood, by what has just been said, to what extent the view here of- fered differs from that of our great pi- oneer geologist. These "lower talcose slates." in all probability, are identical with the intermediate or upper Eozoic beds, just described, and therefore their position rela- tively to the beds beneath them and above them, is the same, whether they be consider- ed upper Eozoic or lower Palæozoic. It will have occured to the reader before this to in- quire what kind of evidence is needed to sep- arat e the lavers of rock of one formation from those of another. Like most simple but comprehensive questions, this cannot be an- swered in a few words, and I will not at- tempt to do more than hint at the nature of the answer in this place. The bases for con- clusions in geology have widened and deepen- ed by long and patient comparisons of obser- vations of different kinds of facts; so that at the present time botany, zoology, mineralogy, topography, and a thousand other things are cited as evidences that this or that layer of rock is older or younger than another. But in questions of age all other means of investi. gation are founded upon stratigraphy, which proceeds to study the chronological order in which one rock is laid down upon another, on the assumption that in sedimentary rocks that which is really on top is the later or newer in origin, and that which is below is the older. In rocks where certain minerals or plant and animal remains, called fossils, occur, they have been carefully studied, ' and where various kinds of similar remains have been found, geologists have assumed that the beds containing them were of the "same horizon," even when stratigraph- ical evidence was not at hand to estab- lish the point. Thus the secondary or indi- rect evidence has often been successfully used to correct the apparently direct evidence of superposition where this has been obscured by reason of the overturning of a series of beds, which has caused those first formed to ap- pear, at the present time, on top of newer strata. This much premised, it can now be explained that the one indispensable concom- itant of a change of formation is a "physical

*See Proc. Am. Inst. of Min. Engrs. Troy meeting, 1883. tree note No. 2, at the end.

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GEOLOGY.

break," between the two. That is to say, that as long as the layers of rock appear to be nearly parallel to each other, the strong inference is that the beds are all of the same formation, but where a sudden change of "dip" (or inclination to the plane of the horizon or in compass direction) is noted, especially when accompanied, as it usually is, by a change of the material of which the beds are made, the presumption is strong that this marks the end of the formation to which the lower bed belongs; and that then suc- ceeded movements of the crust of the earth altering their horizontal position ; and that we have the commencement of a new formation, of which the bed, showing the change of dip and material, is the bottom layer.

The explanation of the exceptions to this rule would lead me far away from my present plan, and must be omitted here. It may be found in any elementary book on geology. Now, to apply this to the present case: There are no good exposures of the Hellam quartzite with the slate below it at any place in York County which the writer recalls. On the flank of the South Mountain, the quartz- ite is very much rent and crushed into frag- ments, while of the small patch on the map about two miles west of Case's ore bank (No. 8 on the map) no accurate dip was recorded. The Hellam quartzite, of which a part composes the "Chikis Moun- tain." exhibits indeed, in its numerous fold- ings, the rock called by Rogers "talcose slate" between its two principal beds of quartzite, but not appreciably lower than the latter.

We are forced to look to other parts of the county for a clearer knowledge of the relation to each other of this quartzite and the schists on which it rests. We find abun- dant instances of this contact in Chester County, north of the valley of that name, and in all of them the quartzite lies "uncon- formably" (i. e. with changed dip) upon the schists. The latter, it is true, are somewhat different in minor characteristics from those of which it is here the question, but so also is the quartzite. Yet we have the best rea- sons for believing that each is of contempora- ry origin with its analogue in York County; and indeed, the differences, which would not be considered at all important by any but a critical geologist, are what we might expect when we remember that these rocks are sedi- ments laid down at the bottom of successive seas, and that their characters depended upon the kind of material which different streams, draining different parts of the coun-

try during different epochs, brought down to be strewn out at different localities .*

It will be explained before long that the physical break between the Eozoie schists and the limestone series is rendered highly prob- able by the observations in York County; but that between the flat arch belt and the Hellam Township quartzite must rest upon the direct evidence obtained in other counties, unless here also we may apply the indirect method, mentioned above, and conclude that inasmuch as the Hellam quartzite contains one impor- tant fossil, and the Eozoic schists contain none that have been yet discovered in York County, this fact alone entitles them to be considered different formations.

This fossil is the Scolithus linearis, and is supposed to be the burrow hole left by some boring worm, which inhabited the Hel- lam (or Potsdam) sea. It was first recognized and named by Prof. S. S. Haldeman in the rocks of Chikis, and has since become one of the most characteristic marks of the lower Palæozoic series, and one of the few widely distributed fossils of this formation.

The Hellam or Chikis quartzite is a hard quartzose rock of which the general color is white or gray, tinted by some other color, usually pink, brown, or blue, depending up- on the minerals with which it has been associated. It is almost always crystalline, and in disturbed regions like this, is most frequently found in broken fragments rather than in continuous beds, owing to its brittle- ness, which prevented it from yielding grad- ually to the strain which has folded and tilted the other rocks of the county. These strains have twisted, broken and crumbled it; but on account of its great hardness and its resistance to the chemical action of the atmosphere, it is the least altered or decom- posed of all the rocks to be considered here, and almost always indicates its presence by a hill, whatever be the position of its strata. t

It is not necessary to specify the localities within the county, where this quartzite oc- curs, because they are indicated by yellow on the geological map; still less is it desirable to discuss here all the possibilities of structure which these scattered out-crops suggest. It is important, however. before leaving the floor of the Paleozoic column to say that eleven years of experience in the field have caused me to doubt the correctness of ascrib-

*Let any one observe the great differences between the char- acters of the sand beach of our own Atlantic coast within short distances. See on this subject Delesse's important contribu- tion entitled "Geologie du Fond des Mers," and the writer's no- tice of the same in Proc. Am. Philos. Soc.

+The reason of this is that the erosion which has torn oft hundreds and perhaps thousands of fect of the other measures has not been able to reduce it to the same extent, and it remains consequently as a hill or chain of hills.

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HISTORY OF YORK COUNTY.

ing to this formation the iron ores which are found in the schists immediately above the quartzite .*

The Grubb ore bank (No. 111 of the map) is the only one which lies wholly with- in the area of the Hellam quartzite, as given on the map, but a reference to the disposi- tion of this bank (Vol. C, p. 64, 2d G. S. of P.) leads to the belief that the larger part of the ore lies in a small remnant of the bottom schists of the next higher forma- tion, which has escaped the erosion that cut off the higher layers of that formation. Part of it, however, answers to the description of an iron ore which may really belong to the quartzite, and which has been noticed in the rocks forming the outer casing of the South Mountain. t

THE YORK LIMESTONE (AURORAL OF ROGERS).

In part the Calciferous Sand Rock of the New York Survey). This important member of the Paleozoic series in York County con- sists of at least two and perhaps three dis- tinct kinds of rock, and inasmuch as the kind that occurs at the bottom (resembling strong- ly that which occurs among the limestone beds themselves, and also above them) has already been mentioned several times by an- ticipation, it will be advisable to consider it first.

HYDRO-MICA SCHISTS.

It was previously stated that Rogers, and following him, almost all other writers on geology up to the commencement of the sec- ond geological survey of Pennsylvania, had given the name of "talcose slates" to a group of rocks which he connected in epoch with the quartzite. The word talcose was applied to them because, from their softness and greasy feel, it was assumed that they were largely composed of "talc," which is a sili- cate of magnesia containing water. But subsequent investigations of these rocks in the chemical laboratory have shown that they contain little or no magnesia, and that they derive their peculiar characters from large amounts of a group of micas contain- ing potash or soda and water. Prof. James D. Dana conceived the happy thought of naming the group the "Hydro-micas" or water (containing) micas, and naturally the

rocks which mainly composed them are called Hydro-mica* schists.

These hydro-mica, or nacreous schists, are not of uniform appearance. Sometimes, and especially in the beds that underlie the lime- stone, they are firmly compacted together, making hard rock masses and high hills, as at many places along the Susquehanna from Wrightsville to Cabin Branch Run, and elsewhere in the county. Sometimes they are so much disintegrated as to form dust, which on close view is seen to be mainly made up of little glinting particles. In the former case the beds are very often strewn with little yellow crystals of metallic lustre, composed of the sulphide of iron and called pyrite by mineralogists, or mundic by the miners. Again, in place of these little crystals of iron (and occasionally copper) sulphide are beautiful casts or moulds, of the shape of cubes, more or less filled with a dark brown iron rust obtained from the de- composition of the original crystals. These little crystals have been of no small impor- tance to the prosperity of York County, for there is good reason for believing that by far the largest part of its iron ores have been derived from their oxidation, transportation by water and final deposition in the clays formed from the grinding up of the rocks which originally contained them.t

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History of York County, Pennsylvania : from the earliest period to the present time, divided into general, special, township and borough histories, with a biographical department appended, Part 92

Author: Gibson, John, Editor Publication date: 1886 Publisher: F.A. Battey Publishing Co., Chicago Number of Pages: 1104

Author: Gibson, John, Editor
Publication date: 1886
Publisher: F.A. Battey Publishing Co., Chicago
Number of Pages: 1104