USA > Massachusetts > Plymouth County > Hingham > History of the town of Hingham, Massachusetts, vol 1 > Part 4
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Intermediate between Andrew Heights and Kilby Street, by a reduction of the level of the road over an elevation, and the neces- sary excavation of rock, there is left exposed on the left side a cliff of considerable interest. The main body is a dark diorite, but there may be seen by close examination a distorted dike of felsite eight or ten inches in width, and a mass of diabase trap, both of which have been intruded into it. The trap contains an
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The Geology of Hingham.
unusual quantity of sulphide of iron in crystals, as may be seen in specimens from this locality deposited in the general collection of the Public Library.
The diorite of the area of this rock under consideration does not follow on East Street beyond Kilby, as its trend which is northeast and southwest, leads to its development along the latter street which has the same direction, and where it is found show- ing itself on the road and adjoining lands at various points for more than half a mile. At the junction of East and Kilby streets it follows the curve from the former to the latter directly in the roadway. Passing northeast on Kilby, it may be observed in lim- ited exposures on the left of the road until the crossing of the rail- road is reached, where there is a lateral extension of it 300 feet west on the line of the rails, and 150 feet east. About 500 feet from the crossing it again appears on the left side of the street, followed at a short distance by granite. On the right side of the road at 940 feet from the railroad may be seen a rock exposure presenting a face towards the street of about thirty feet, the first portion of which for twenty feet is diorite, the rest being granite. Proceeding 420 feet more along the road there will be seen ledges on the left side back from the street which extend for a further distance of about 270 feet. These are all diorite. After passing these 130 feet, there may be observed on the right of the road, and just beyond the fence which borders it, a face of rock about twenty feet in width, the first portion of which, about one third, is granite, and the rest diorite. The two rocks are separated by a diagonal line having a declination of 45° N. E., thus showing the latter rock as resting somewhat upon the former. Just beyond this exposure granite follows for a distance of about 70 feet. There are no fur- ther exposures of diorite on the road towards Rockland Street, the few outcrops of rock observed there being all granite.
Another area of diorite exists near the eastern border of the town towards Cohasset, showing itself extensively on Weir, East, Side Hill, and Hull streets.
On Weir Street going from East Street, there is scarcely any other rock observable for at least one third of a mile. Beyond this, it alternates more or less with granite for about one eighth of a mile, when it gives place entirely to the latter. On the east side of the street, 2,310 feet from East Street, there is an exposure of rock presenting a face to the carriage-way, showing a singular mixture of both diorite and granite.
This will be again and more particularly referred to in remarks to follow upon " mixed rocks,"-a name given by Professor Crosby in treating of a like association observed by him at Marblehead and Salem.
As a general fact, it may be stated here that the diorite of Weir Street is not so clearly typical as that of East and Kilby streets before described. A preponderance of the feldspar and partial decomposition, gives it in some cases a dirty white exterior.
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History of Hingham.
On East Street, passing from Side Hill Street towards Cohasset, may be found in the fields adjoining the right side of the road and back from it many extensive ridges of rock. Short of 200 feet a small exposure of granite occurs just within the fence-wall, and a little further on, say twenty or thirty feet, is one of diorite. Back of these about sixty feet is another of diorite. Following the road 310 feet from these, rock appears in patches over the sur- face for cighty feet, extending some distance back from the fence, which is likewise diorite. Passing beyond these exposures 140 feet, fields of rocks are reached occupying a great part of the sur- face for at least 700 feet. Some of them are diorite, some granite, while others among them, presenting surfaces of both diorite and granite, are apparently of the singular combination mentioned as mixed rocks. It would, however, be necessary to blast them in order to verify this.
On the left side of the road there are but few exposures, and these are of the same general character as those mentioned.
On Side Hill Street, and in fields adjoining, diorite appears abundantly. At a distance of 310 feet from East Street, a small exposure may be found on the right side of the roadway, and fifty feet beyond this another just within the border fence. Proceeding 300 feet further there is within view on the fields at the right many ridges and smaller rock masses extending over an area of two or three acres, all or nearly all of which are of the same rock. On the left of the road, 620 feet from East Street, a long ledge of rocks skirts the carriage-way, which extends 280 feet. For the first few feet it is diorite, the rest of it is granite. Granite is also the pre- vailing rock on the high ground of the adjoining field.
When entering Hull Street from East Street, diorite appears on the right side, close to the junction of the two streets, both on the border of the roadway, and within the enclosure of the adjoining estate. Proceeding on Hull across the railroad, a high cliff of rock is seen back from the street; 200 feet or more in length, which exhibits upon some portions surfaces of diorite, upon others granite, - showing it to be probably of the mixed character mentioned in previous cases and to be described hereafter. This is succeeded by granite, and there is no more exposure of diorite on or near the street until about 2,060 feet from the railroad, where a ridge of it appears on the field at the left, not far from 200 feet from the fence ; and 300 feet farther some may be seen on both sides of the road. The rock exposures beyond these are all granite, until after passing Canterbury Street between three and four hundred feet, when there may be seen ridges on the fields skirting the left side of the road and extending for a quarter of a mile or more, which are likewise of the mixed diorite and granite. The rocks of the last 300 to 400 feet of the street within the town limits are all granite.
All the exposures of diorite within the territory of the town that may be observed in passing along the streets and lanes have been
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The Geology of Hingham.
mentioned, except a limited one on Central Street, between four and five hundred feet from Elm Street, in a field adjoining the west side of the road, and two others of small area on a field at the corner of Central and Elm streets. Away from usual travel between Fort Hill Street and Weymouth River the rock appears in numerous exposures. Reference to the map will give their location.
MIXED ROCKS.
The rocks so-called by Professor Crosby, though simply com- posed of a mixture of the two kinds already described, are of such peculiar combination as to seem worthy of notice under a separate heading. There is no appearance among them, as far as observed, of anything like a dike of either penetrating the other. There is found simply a mixture of masses of every size and shape, each single mass being clearly distinctive as granite or diorite, the ele- ments of one in no case coalescing generally with the other. The locations of these rocks have been mentioned in the remarks upon the diorite.
There seems no way of accounting for such mixture except by supposing that at the time of their cruption the rocks existed sep- arately beneath the surface in two contiguous zones, both being in a plastic condition, and that when forced to the surface they were made to intermix so as to present themselves as now found.
PETROSILEX.
The rocks of Hingham hitherto known as porphyry, compact feldspar, and felsite, the writer classes under the name of petrosi- lex, as with but one or two exceptions to be mentioned, all belong to that division of such rocks as contain over 63 or 64 per cent of silica, and which Phillips and others have designated as petrosilex, retaining the name felsite for those of a more basic character, and having a plagioclase feldspar instead of orthoclase as a constituent.
The name "porphyry " is no longer in use as a substantive by geologists. It was applied by the ancients to rocks generally ho- mogeneous, but which contained crystals, commonly feldspar ; and this use continued to modern times. As, however, the rocks so- called differed widely in composition, and it became necessary in the progress of science to define their character more particularly, the name became obsolete. The word " porphyritic," however, remains in common use as an adjective expressing the texture of rocks of a homogeneous base, having crystals disseminated through- out their mass. Thus petrosilex with enclosed crystals is called porphyritic petrosilex, and diabase, the rock of trap dikes with enclosed crystals, is called porphyritic diabase or porphyritic trap.
The writer, in communications to the Boston Society of Natural
30
History of Hingham.
History and otherwise, has expressed the opinion that much of the petrosilex of the Boston Basin, and particularly the red rock of Hingham, was derived from conglomerate. This view is not held by others, whose opinions are entitled to respect ; but this has not shaken confidence in his own. There is petrosilex, however, in Hingham of quite a different character, but which he claims has another origin. Mention of that will follow some further remarks upon the red variety.
Prof. Edward Hitchcock, in his great report upon the " Geology of Massachusetts," mentions under the head of Porphyry the red rock now under consideration, as occurring in Hingham in ridges a little north of the village. Undoubtedly this accurate observer found such ridges, though but one small exposure can now be found above the surface. This is near the junction of Crow-Point Lane and Downer Avenue. Masses of this beautiful rock may be seen in the stone walls of Lincoln Street near Thaxter, and sug- gest to the mind that in widening this street for the greater con- venience of travel the ridges noticed were destroyed.
The rock is called above beautiful. Professor Crosby speaks of it as the most beautiful of any in Massachusetts, and it undoubt- edly is so. The color is a bright red, with interspersed spots of lighter or darker hue. The variation was caused apparently in some cases from the enclosure of pebbles, which, with the general mass, became more or less fluent. The pebbly structure can be better seen on weathered surfaces than on those caused by recent fracture.
The other variety of petrosilex referred to above, differs essen- tially from the red, being of different color, rather more glassy in lustre, entirely homogeneous, and presenting no appearance indi- cating enclosed pebbles. Of the origin of this variety there can be no question. It has the chemical constitution of granite, oc- curs associated with it, and is undoubtedly the same with granite, excepting that its mineral constituents are not crystallized, the rock being too rapidly cooled to admit of crystallization. This variety is always in Hingham associated with the granite. It may be found with the granite that forms the cliffs of Peck's Pasture, bordering the Home Meadows, and also on Lincoln Street, in the rear of the first house next north of the Unitarian Church which faces Fountain Square. Specimens from these and other localities are in the collection of the Public Library, where may also be seen those of the red variety.
PORPHYRITE.
The name Porphyrite has been given to basic rocks differing but little in composition from Diorite and Diabase. Like them they are composed of a triclinic feldspar with hornblende or augite, but they are not, like them, crystalline granular. They
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History of Hingham.
History and otherwise, has expressed the opinion that much of the petrosilex of the Boston Basin, and particularly the red rock of Hingham, was derived from conglomerate. This view is not held by others, whose opinions are entitled to respect ; but this has not shaken confidence in his own. There is petrosilex, however, in Hingham of quite a different character, but which he elaims has another origin. Mention of that will follow some further remarks upon the red variety.
Prof. Edward Hitcheock, in his great report upon the " Geology of Massachusetts," mentions under the head of Porphyry the red roek now under consideration, as occurring in Hingham in ridges a little north of the village. Undoubtedly this accurate observer found sneh ridges, though but one small exposure ean now be found above the surface. This is near the junction of Crow-Point Lane and Downer Avenue. Masses of this beautiful roek may be seen in the stone walls of Lincoln Street near Thaxter, and sug- gest to the mind that in widening this street for the greater con- venience of travel the ridges noticed were destroyed.
The rock is called above beautiful. Professor Crosby speaks of it as the most beautiful of any in Massachusetts, and it undoubt- edly is so. The color is a bright red, with interspersed spots of lighter or darker hue. The variation was caused apparently in some cases from the enelosure of pebbles, which, with the general mass, became more or less fluent. The pebbly structure can be better seen on weathered surfaces than on those caused by recent fracture.
The other variety of petrosilex referred to above, differs essen- tially from the red, being of different color, rather more glassy in lustre, entirely homogeneous, and presenting no appearance indi- eating enelosed pebbles. Of the origin of this variety there can be no question. It has the chemical constitution of granite, oc- curs associated with it, and is undoubtedly the same with granite, excepting that its mineral constituents are not crystallized, the roek being too rapidly cooled to admit of crystallization. This variety is always in Hingham associated with the granite. It may be found with the granite that forms the cliffs of Peek's Pasture, bordering the Home Meadows, and also on Lincoln Street, in the rear of the first house next north of the Unitarian Church which faces Fountain Square. Specimens from these and other localities are in the collection of the Public Library, where may also be seen those of the red variety.
PORPHYRITE.
The name Porphyrite has been given to basie rocks differing but little in composition from Diorite and Diabase. Like them they are composed of a trielinie feldspar with hornblende or augite, but they are not, like them, crystalline granular. They
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The Geology of Hingham.
contain from 56 to 58 per cent of silica. Rocks of this character, of various shades of color, are found at Nantasket, but only one exposure has been noticed in Hingham. This is on the shore of the marsh land that borders Weir River, quite near Rocky Neck. At this place it is of a dark-brown color, similar to that of the brown sandstone commonly used in structures of Boston and New York. It is a heavy, tough rock, and undoubtedly owes its origin to volcanic action, being, like melaphyr, an ancient lava.
DIABASE.
Diabase, like Diorite, is composed partly of a triclinic Feldspar, generally Labradorite, but differs from it otherwise in having Augite associated with it instead of Hornblende. Not unfre- quently Mica is found in its composition, and often Pyrite, though these are not essential ingredients. This rock is generally known as Trap, and the dikes which it forms in all parts of the town are called Trap Dikes. The rock as exposed at the surface exhibits more or less the results of decomposition, becoming of a dull green color, from the change of the Augite to Viridite. It has a much higher specific gravity than granite, and is exceedingly tough. The bluish, close-grained masses often found in the soil and called Blue Rocks are of this kind.
Diabase forms dikes alike in the granitic rocks of the town, and in those of the Slates and Conglomerates to be hereafter mentioned.
DIKES.
Having now noticed all the rocks of the Crystalline series found in Hingham, - Granite, Diorite, Petrosilex, and Diabase, - and as each of them is found in dikes within the Boston Basin, two of them at least in Hingham, it seems fitting to present here some special remarks upon the form of structure known under that name, and to give an account of localities where they may be observed.
Dikes are igneous, unstratified rocks, which occupy fissures in the formations, and which have been forced up from beneath the surface of the earth in a liquid or semi-liquid state, into the superincumbent rocks.
This molten material undoubtedly at first spread itself, as does the modern lava of volcanoes, over considerable areas after reach- . ing the surface. As seen in Hingham, the rock of the dikes is usually found only within the walls of the fissure that gave it pas- sage, the decomposition and washing away of the hundreds of feet of solid matter that once formed the surface having generally left for our view only what is now seen within narrow limits. Often, in forcing a passage through the invaded rock, masses of the latter
32
History of Hingham.
were torn off and enclosed in the molten matter, and it is not therefore uncommon to find in Hingham instances of the enclos- ure of granite within the darker trap rock of the dike.
The name Trap has been generally used to designate the dark- green or black rock forming dikes ; but as it is now recognized that different rocks of like appearance constitute the invading ma- terial, it is necessary to be more definite in scientific description. The dikes of Hingham as far as examined, with two or three ex- ceptions only, are all of Diabase.
In narrow dikes the rock has a homogeneous structure, as the sudden cooling prevented a crystallization of its mineral constitu- ents, but in those of any considerable width where the material cooled more slowly, it is often porphyritic towards the central portion, crystals especially of feldspar being disseminated. Upon the invaded rock the action caused by the introduction of the molten matter is generally more or less perceptible by a change in its structure near the junction of the two rocks, and frequently by the production of minerals along their margins. In Hing- ham, Epidote is not uncommonly found as the result of this action. Mention will now be made of some of the dikes which have come under the observation of the writer.
MEETING-HOUSE HILL, MAIN STREET, SOUTH HINGHAM. - There is a dike in the granite of this elevation but a few steps north from the church which may be seen on the surface of the rock and traced sixty to seventy feet to the margin of the carriage road. It is from five to six feet in width, and runs in a northwest and southeast direction. Generations of men have come to the temple here to worship, wholly unconscious that their footsteps were over a record of events that took place millions of years before man breathed the breath of life.
LEAVITT STREET AND JONES STREET. - Between these two roads on land of Mr. James Jones is a rocky hillock of granite about equidistant from both, in which may be found three trap dikes not far apart, one of which has the considerable width of ten feet. To readily find these, proceed from the bridge that crosses Weir River 700 feet in a southeasterly direction on Leavitt Street, which will bring one to Mr. Alanson Crosby's house on the left side. By passing to the rear of the house about 300 feet from the road, the rocks will be reached with their en- closed dikes. The most northerly of the three is about two and a half feet in width, the second, eighteen feet from the first, is ten feet wide and exposed for a distance of seventy-five feet. These two show well on the face of the granite cliff which en- closes them. The third, forty feet from the last-mentioned, is from three to four feet wide. This will not be readily perceived without close examination, as it is only on a comparatively level spot and obscured somewhat by surface soil. The direction .of these dikes is east and west. Two hundred and fifty feet, more or
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The Geology of Hingham.
less, east of these dike exposures occur considerable bodies of trap, but the connection with them is not perceptible.
On Leavitt Street, about a mile and a half from Leavitt's Bridge going east, and less than a quarter of a mile before reaching the town line, a trap dike crosses the road diagonally. It appears first on the right side for a few feet, and the exposure on the left is seventy-five feet from where the first is lost to view. In neither place does it show above the surface more than a few feet, nor can it be traced beyond the two exposures. Its width is about six feet, and it is porphyritic. Its direction is east and west.
LASELL STREET. - Considerable elevations of granite skirt Lasell Street on the left side, some of which approach and border the high- way. After passing Free Street 740 feet, one of these is reached, which presents a bold front, having a very interesting dike of about six feet in width. Lichens obscure this somewhat, on the face of the rock as seen from the street, and one needs to climb to the upper surface to study it to advantage. Here it is found extending itself a considerable distance east, showing, away from its margins, a porphyritic character, the crystals of feldspar being quite distinct. Fifty feet south of this is another dike, parallel with the first, but having a width of only thirty-two inches. This does not exhibit crystals of feldspar so perceptibly, its cooling having been too rapid for their favorable development. This dike cannot be seen from the street, as the front face of the rock has retreated from its border. The two dikes have both an cast and west direction by compass, as have nearly all that are found in the granite not approximate to the rocks of the sedimentary series.
LONG BRIDGE LANE. - At the granite quarry of Mr. Israel Whitcomb, about a quarter of a mile from Union Street, may be seen two dikes east and west by compass, one about a foot wide, the other twenty-two inches. They are not far from thirty feet apart.
FRIEND STREET. - On the right-hand side of this street, pro- ceeding from Main, and not far from the latter, may be seen two dikes cutting through the granite of the roadway, both having a general direction of east and west, and both of which may be traced for considerable distances. The first is found 330 feet from Main Street, and varies from four to six feet in width. This may be observed in the adjoining field, 80 to 100 feet east from the road, and has been traced west across meadow land in differ- ent ledges, nearly 1,000 feet. The second one is about forty feet beyond the first-mentioned, and has a width of about two feet. It appears on both sides of the carriage way in the bordering ledge through which the street was cut, but is not so readily seen on the left as on the right without close observation. This has been traced 120 feet or more.
UNION STREET. - There is a dike on this street, 360 feet from Lasell Street going east, which may be seen in a ridge of granite VOL. I .- 3
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History of Hingham.
which extends along the left side of the road for a distance of about 120 feet. It varies in width from fifteen inches to nearly two feet, and is much distorted. The general direction is, how- ever, east and west. It may be traced nearly the whole length of the ridge.
About 2,000 feet beyond this, going from Lasell Street, another dike occurs which crosses the street diagonally. It may be seen on both sides of the roadway in the granite, and may be traced into the adjoining field on the left seventy-five feet or more from the fence. Its width is about three and a half feet; its direction east and west. See Figure No. 1. The crosses (xx) represent exposures of the granite.
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