USA > Pennsylvania > Delaware County > History of Delaware county, Pennsylvania, from the discovery of the territory included within its limits to the present time > Part 46
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Below the " water shade" the underlying rocks are so much covered with dilu- vium, and the earthy matter resulting from their own decomposition, that it is exceedingly difficult to trace the direction of their strata with any great degree of certainty. North of Darby Creek Ferry we find the strata micaceous, and in one belt of considerable dimensions, the place of the mica is nearly altogether supplied by dark-colored kyanite.
In the immediate vicinity of Darby Creek Ferry, and several other places in .
the same neighborhood, the exposures of large granitic veins, of the very coarsest materials, are numerous. One of these is on the island of Tinicum, half a mile above the ferry. In all these reins feldspar is the most abundant material.
Below the post road, and near Little Crum Creek, the gneiss strata become more solid, the proportion of quartz increased, and the mica, which is dark- colored, less abundant. Here quarries have been opened, from which large quantities of stone were taken to the Delaware Breakwater. These quarries are not in the range of strata in which nearly all the well-known gneiss quarries of Delaware county are located.
Near Lieperville, but still southeast of the regular gneiss quarry range, are found strata of no great dimensions, but abounding in silicious particles, almost to the exclusion of the other ingredients of gneiss. These particles are sharp and gritty, and some years ago large quantities of the partially decomposed rock were manufactured into scythe stones, and were well known under the name of " Crum Creek Stones."
The large trap dyke that has been mentioned, disappears in Springfield, and the southwestern part of that township, with the northwestern part of Ridley, the southern part of Nether Providence and the eastern part of Chester town- ships being, in a great measure, free from large intrusive veins of granite, the gueiss is there found undisturbed and unaltered, and generally of an excellent
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GEOLOGY OF DELAWARE COUNTY.
quality. It is within the limits that I have mentioned that all the noted gneiss quarries of Delaware county are found. Isolated patches of a similar rock that have escaped the influence of secondary metamorphic agencies, are met with in other parts of the County, but they are of limited extent, and being inconvenient to tidewater, quarries opened in them can only serve for local purposes.
The quarries on Crum and Ridley Creeks are on or near tidewater, affording every facility for transporting their products to market, and for which the city of Philadelphia, almost from the time it was founded to the present time, has afforded one both convenient and ready.
Though the structure of these rocks, and the minerals entering into their com- position, bring them up to the standard of true gneiss in the strictest sense of the term, still the usual varieties of that rock are found in the different quarries. On the one hand it approaches granite so nearly as not readily to be distin- guished from that rock, and is known in commerce under the name of granite, while on the other hand it similates mica schist, retaining, however, its qualities of hardness and durability. The former is cut and employed for every architec- tural purpose for which granite is adapted, while the latter, besides being exten- sively quarried as a building stone, supplies nearly all the curb-stone used in the city of Philadelphia, and some for other places. For this latter purpose no stone could be better adapted ; its hardness and toughness fitting it for the severe usage to which it is subjected on the sides of the streets,-while numer- ous seams, nearly at right angles with its cleavage, and at suitable distances from each other, enables the skilful workman to take out blocks of nearly the exact size wanted, and of lengths varying from five to forty feet.
Immediately west of Ridley Creek, and for some distance above the post road, and probably for some distance below it also, the character of the rock is essen- tially different from that in its immediate vicinity. As exhibited in Spencer Mellvain's quarry above the road, it is substantially composed of hornblende and quartz intimately blended, and has a specific gravity of 3.13.
We have now considered the underlying rocks of the southeastern section of the county, below the development of trap first mentioned, and an imaginary continuation of it to the neighborhood of Chester. On the upper side of this trap much less change has been effected in the character of the adjacent strata : still a change is well marked in many places. In proceeding northward from the line already designated, the strata gradually become micaceous, until they pass into well marked mica schist in the eastern part of the County. Before reaching this point, strata of hornblende schist and silicious schist, alternate with micaceous gneiss, or with true mica slate. In the southeastern part of Marple township, a few narrow strata occur, composed chiefly of a sharp sili- cious sand, with minute particles of mica interspersed. The lithological charac- ter of this rock, as well as its immediate associations, would entitle it to be classed as a mica slate. It is from these subordinate strata that the well known " Darby creek scythe stones " are manufactured .*
The map will exhibit approximately where gneiss predominates on the one hand, or mica slate on the other; but it may be repeated that the colors of the map only indicate the predominance of a rock, and not its presence, to the exclu- sion of all others.
The next belt of strata to be noticed is that in which all other rocks may be considered subordinate to mica slate. This belt occupies quite a considerable district in our County, but has attracted little attention, from the fact that a large part of it is hidden from view by the remains of an ancient deposit of clay, sand, and gravel that rests unconformably on the micaceous strata which compose it. This deposit has mostly disappeared along the streams ; which circumstance allows us to form a good judgment of the general character of the underlying rock, though its particular features in many places must remain a matter of conjecture.
For the boundaries assigned to these micaceous strata, which are in a measure
* The stone, after having been removed from the quarry, is split into blocks of nearly the right size These are dressed into shape with a kind of hatcher, after which the stones are finished by grinding them on a large stone running horizontally, with pewter sand and water.
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GEOLOGY OF DELAWARE COUNTY.
arbitrary, the reader is referred to the map, where it will be seen that they occupy the largest part of Haverford and Marple townships, a considerable scope of Springfield and of Nether Providence. Within this scope, intrusive veins of granite have effected a considerable change in the character of the rock in seve- ral places. These may be seen near Leedom's Mill on Cobb's Creek, and near Darby Creek, on the Philadelphia and West Chester Plank Road. On the east side of the latter creek, one or two of these coarse granitic veins have been beautifully displayed by the cutting made in grading the hill. What is singular, the strati- fied rock adjoining the veins is very much decomposed, while the veins them- selves have undergone but little change.
The mica slate is remarkably garnetiferous in some places, while in others very few garnets are to be seen. They may be seen in the greatest abundance east of Haverford Friends' Meeting-house, towards Cobb's Creek, where, in the decomposed mica slate forming the bed of the road from the Catholic Church to the creek, they appear like a dark-colored gravel ; but upon inspection each specimen will be found to be a perfectly crystallized garnet. The mica slate near the foot of the hill, which contains imbedded garnets, is a light-colored, flaky variety, but the sandy bed of the road resulting from its decomposition is of a reddish color. This is supposed to be caused by the slow decomposition of the garnets themselves.
The mica slate is garnetiferous on Darby Creek, Naylor's Run, and in other places where the rock is exposed. The garnets are generally small-from the size of a shot to that of a pea, but not unfrequently those of a larger size are found.
Occasionally this rock takes in a little kyanite as a component ingredient ; but this is rare, and when it does occur, the kyanite is of a much lighter color, and in much less proportion than that found under similar circumstances near Darby Creek Ferry. Surface specimens of rock thus constituted may be seen about a half mile south of Haverford post-office.
Quartz rock so usually accompanies mica slate, in subordinate strata, that I need hardly mention that our district forms no exception to the general rule. Large exposures of this rock in place, are found in Marple township.
In several localities, the mica slate assumes the crimped and folded condition that forms one of its striking characteristics. This is well exhibited on the hill east of Beatty's Mills ; near Lowne's Run, on the road from Springfield to Chester, and in several localities in Marple township.
Mr. Rogers extends this Geological district beyond the Brandywine; but most of the strata composing it lose their micaceous character in proceeding south- ward and westward from Crum Creek, though some of them regain that character again as they approach the first mentioned stream. The map will exhibit ap- proximately the character of the rocks there as elsewhere in the County. It may be mentioned, however, that in the section formed by the Brandywine, some of the strata are highly micaceous, and contain garnets, while others have the character of true gneiss, or hornblende, or feldspathic gneiss.
In Birmingham township, a small bed of highly crystalline limestone is found resting in a synclinal trough of the gneissic strata. It has been worked. but it lies so deep, and the quarrying of it is so much interfered with by water, that the further working of the quarry has long since been abandoned. Mr. Rogers does not regard this limestone as belonging to his gneissic formation, but includes it in his primal division of Palæzoic rocks. It was known to older Geologists as " Primary Limestone."
In the southwestern part of the County, the mica in the gneiss rock is dark- colored, and frequently is replaced by hornblende. Some distance up the river there are several trap dykes, from which numerous boulders have been ejected and spread around in such numbers and to such an extent, as to have given rise to the idea that they were brought from a distance, and belonged to a formation known to Geologists as drift. As the presence of these boulders can readily be explained from local geological phenomena, the evidence of which is undoubted, it is unnecessary to resort to any other theory to explain their presence. These boulders are spread over a considerable portion of Bethel, a part of Upper Chi-
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GEOLOGY OF DELAWARE COUNTY.
chester, and extend into Delaware State. On the middle branch of Naaman's Creek they occur in the greatest profusion.
In the vicinity of Village Green, ordinary micaceous strata are observable. Further north, in Aston, above the "water shade," the gneiss strata are highly metamorphic, and appear in great confusion. The plutonic agency that caused this metamorphism (doubtless protruded trap), has elevated hills of some height, and has given a general unevenness to the surface of the country. Still further north in Aston and Concord the gneiss is less altered, and is not characterized by anything remarkable.
Purposely passing over without consideration the several exposures of serpen- tine in our county, with the view of considering them and their accompanying rocks together, I will now proceed to notice the gneissic strata lying west and northwest of the more micaceous belt that has just been noticed. These strata are observed to be in a highly metamorphic condition in many places, and the cause of this metamorphism is readily found in the numerous exposures of trap rock that meet the eye in many places. This gneissic belt occupies nearly the whole of the townships of Radnor, Newtown, Edgmont, Thornbury, and parts of Upper Providence and Middletown. A part of Radnor township chiefly north of the Gulf Creek, Mr. Rogers includes in the lowest member of his ancient Palæozoic strata. It will be noticed hereafter. Beyond Radnor, westward, tlie northern boundary of the gneissic district, now under consideration, passes out of the county north of Newtown township.
The trap rock throughout this region is much finer grained and more compact than the same rock found in and near Upper Darby. Mr. Rogers, in his report of the State survey, notices a very large dyke commencing some distance west of the Schuylkill, crossing that river at Conshohocken, and terminating in Delaware county " near the road leading from the Lancaster Turnpike to the King of Prussia village." The author has examined this dyke at Conshohocken and at the Gulf Mills in Upper Merion, where there is a good exposure of it, and he labors under a great mistake if he has not seen the same dyke, or very large branches from it, much further southwest than the point designated as its west- ern termination. Be this as it may, such dykes with innumerable smaller branches exist, and that they have been the chief instruments in breaking up the strata of this district of country, and in giving the rock its present meta- morphic character, cannot admit of a doubt. My friend, the late John Evans, of Radnor, before the publication of the State survey, held the opinion that the trap dykes extending to the southern part of that township, (some of which passed near his residence,) were branches of the main dyke passing the Gulf Mills.
This trap is of a remarkably fine compact texture, especially in the neighbor- hood of Siterville, where it breaks with a purely conchoidal fracture.
The gneiss of this district, where it has not been too much altered by the trap dykes, does not differ materially from the gneiss of Upper Darby already noticed, although none has been noticed so perfectly porphyritic. Mr. Rogers says the prevailing varieties are, "first, a massive feldspathic gneiss, some of it mica- ceous, and some of it like a stratified syenite; and, secondly, a dark, hard, hornblende feldspar gneiss, thinly laminated and strongly striped when viewed in transverse section." Mr. Rogers thinks he has discovered " a remarkable feature in the uppermost or northern bands of gneiss * ** * * which next adjoin the base of the primal series, in the possession of a less than usual com- pleteness of crystallization in the constituent minerals." Mr. R. has acknow- ledged the difficulty of tracing the dividing line between the two formations, and as he had a theory to support, which this imperfect crystallization favors, it may be possible that his specimens were obtained on the wrong side of the dividing line.
Besides the varieties of gneiss mentioned by Mr. Rogers, as occurring in this district, there is one, not noticed by him, and not found in the southern or first described district. Its peculiarity consists in its quartz possessing a light blue color. This occurs in a belt of considerable breadth, which the author has traced from the eastern part of Radnor to the eastern part of Willistown, in
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GEOLOGY OF DELAWARE COUNTY.
Chester county. This blue quartz is best seen in the decomposed rock where its small grains form an angular gravel. Specimens weighing several pounds are sometimes met with.
North of the gneiss of Radnor the rock is chiefly a talcose slate, containing much quartz and is in a highly metamorphic condition. This forms what is known as the " Gulf Hills." At the foot of these hills, near the Gulf Creek, a small deposit of impure limestone exists within the limits of our county.
A notice of the numerous exposures of serpentine within the county, only the larger of which will be particularly described, will close our account of the un- derlying rocks The map will show, with a good degree of accuracy, the loca- tion of each exposure.
Mr. Rogers has overlooked every out-crop of serpentine within the limits of our County, except one, which is about half a mile from Morgan's Corner, in Radnor township, and which is very inconsiderable in extent. This exposure is 300 feet wide and contains, according to Mr. Rogers, " true injected or igneous ser- pentine, and serpentinous steatitic tulc-slate." It is not believed that this serpentine has any connection with any other exposure of the same rock within the County. There is, however, a small exposure in Lower Merion township, just beyond the Radnor township line, that appears to be continuous with the extensive develop- ment on the borders of the townships of Radnor, Newtown and Marple, and serves to connect this serpentine with the serpentine and steatite of Mill creek, and that of the Schuylkill at the soapstone quarry. The development mentioned is located on the line of separation between the micaceous district and the northern gneissic, and for a considerable distance forms the boundary between them. This development of serpentine embraces both the stratified and unstra- tified rock, and is associated with steatitic rocks, and also with true talcose slate in small quantities.
As is the case with every extensive development of serpentine in the county, this one is accompanied with a trap dyke. A little northwest of the residence of Henry Hipple, in Marple, this dyke at one point is beautifully exposed, and in conjunction with another unstratified rock, highly crystalline in its character, forms a hill of no mean proportions. The fresh fracture of this rock is of a dark green color, and its crystals are so interlaced as to give it such a degree of toughness as to render its fracture very difficult. Its appearance is intermediate between pyroxine and tremolite. Prof. Booth, of the U. S. Mint, judging from a hand specimen, has rather doubtfully decided to call this rock diallage, a variety of augite, while J. C. Trautwine, Esq., who visited the locality, unhesitatingly named it anthophyllite, a variety of hornblende. The distinction between augite
and hornblende is very slight. Whichever of the names may be the more ap- propriate, the vast extent of the rock, will be a surprise to geologists and mine- ralogists who may visit the locality. Diallage is not abundant in this country, while anthophyllite has been regarded as rather a scarce mineral. This locality will furnish enough of the material to build a city. The exposure of the rock at the top of the hill is now nearly excluded from view by the dense growth of young timber. It is shown, however, sufficiently well in the annexed litho- graphed drawing. The great number of boulders of trap and of this associated rock that lie scattered over the side of the hill, attest the force with which the matter of these rocks was upheaved.
This new unstratified rock also constitutes a part of the rocky exposure in Edgmont township, well known as Castle Rock, but the connection between it and the trap cannot be so well seen at this place as at the exposure first men- tioned.
The new rock as it appears in the dyke near Hipple's, and in the scattered boulders round about, is strictly unstratified, but isolated rocks very similar in appearance are found adjacent to this exposure of serpentine in which a lami- nated structure is very apparent. I have not met with these laminated rocks in place.
Near the Yellow Springs road, bordering this serpentine, there may be seen a thin stratum of gneiss, very fine in its texture, and of an unusually white color. This rock, upon close inspection, will be found to have taken into its composition a portion of light-colored talc.
Drawn by B.H.Smith.
EXPOSURE OF TRAP WITH DIALLAGE OR ANTHOPHYLLITE.
Bowen &Co. lith Philada
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GEOLOGY OF DELAWARE COUNTY.
Fragments of a rough, porous, silicious rock, generally very ferruginous, pro- vincially termed " Honeycomb stone," are found strewed around this serpentine. Fragments of this unsightly rock are associated with all our developments of serpentine of any extent, and as they are found in the greatest profusion around the serpentine district of Middletown, the rock will be described in connection with that tract.
The minerals of the serpentine will be given in the general catalogue of the minerals of the County, but it may be remarked that in the northeastern part of the exposure under consideration, magnetic iron ore is found in considerable quantities, apparently interstratified with the serpentine rock. In the same vicinity the serpentine is of a more magnesian character.
Proceeding southwesterly towards Crum Creek, the true serpentine disappears, but the talcose strata are extended, and can be traced for a considerable dis- tance in the direction of the " Blue Hill" serpentine in Upper Providence. It is in this extension that some of these strata assume a more steatitic character, but no soapstone has been found of a quality sufficiently good to be quarried for the uses to which this material is generally applied. Thin strata of a green colored talcose slate sometimes alternate with the mica slate of this vicinity.
The exposure of serpentine known as "Blue Hill" is not large, but at the point specially bearing that name, the rock does not present itself in any ques- tionable shape. If any part of the rock near Morgan's Corner can be regarded as " true injected igneous serpentine," the plntonic origin of the serpentine at Blue Hill cannot be doubted. The rock is very much divided by seams, some of which are filled with amianthus.
The next exposure of serpentine in order is the large tract in Middletown. It is the largest development of this rock in the County, and particularly so in its dimensions across the regular strata of that district. Where it is exposed or approaches the surface very nearly, it constitutes what is known as the " Bar- rens " of Middletown,
The "Honeycomb " stones before mentioned are spread around this serpentine in rather inconvenient profusion. Their origin has been a matter of wonder, as they have not been found in place, but are always seen scattered about as a kind of out-liers on the flanks of the serpentine. Their appearance in many instances resemble the slag of a furnace, and if we cannot conclude that they have resulted from the metamorphism of a ferro-silicious stratum of rock by the application of a high degree of heat, accompanied by the emission of gaseous matter, would it be unreasonable to regard them as the slag thrown off from the surface of a mass of melted serpentine ? The apparent laminated structure of some of these fragments of " Honeycomb stone " favor the former theory, but furnace slags fre- quently exhibit the same appearance. It will therefore be safest to regard them as the vitreons slag of a melted mass of serpentine, until they shall have been discovered in situ ; and that too in connection with strata of a proper compo- sition, that by their metamorphism, would result in Honeycomb rock.
The Middletown serpentine is noted for the large amount of chrome it has afforded. Large quantities have been obtained here within a few years past. It is procured in two ways, by mining, or by washing* the alluvial sand of the beds of streams that have passed through the serpentine. The latter is preferred, as the mineral is then obtained in its purity. When obtained in this state, it has the appearance of a shiny black sand, nearly every particle of which when brought under a glass is shown to be a perfect crystal.
Beyond the serpentine proper of this district, the accompanying talcose or magnesian strata extend to Chester Creek, towards which they are very much decomposed.
* Most of this grain chrome is found in the ancient beds of streams mixed with sand and gravel. These deposits are discovered in various ways, and when found, the soil and superin- cumbent clay are removed, and the whole material of the deposit is thrown into strong iron sieves which are worked by hand under water. The sand and chrome graiu pass through the sieve and are deposited in the vessels containing the water The contents of these vessels are then re- moved to the " wash-house," where troughs are provided, through which water is caused to pass with considerable velocity. In these troughs, which have stops in them to create a ripple, the sand is washed out from the chrome grain, having a much less specific gravtiy. The washing has to be repeated three times before the ore is ready to be dried aud packed in barrels for the manufacturer.
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GEOLOGY OF DELAWARE COUNTY.
There are several more exposures of serpentine in the County which require no particular notice. Their size and location are exhibited on the Geological map.
There has now been brought in review before the reader all the underlying rocks of the County, in fact every thing that has the name of rock in common language. But in the language of the Geologist every deposit that constitutes a part of the crust of our planet, however soft and earthy, is a rock, and such deposits often form the most interesting subjects of his investigations.
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