USA > Massachusetts > Plymouth County > Hingham > History of the town of Hingham, Massachusetts, vol 1 > Part 5
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FIGURE No. 1.
OLD COLONY HILL. - Proceeding from the harbor on Summer Street towards and up the slope of Old Colony Hill, there may be seen on the right side just above the surface a small exposure of trap, being part of a dike which passing east is lost to sight by the covering earth, but which again appears just in front of Mr. Bouvé's stone wall, near the corner of Rockland Street. Here it presents a flat face upon which may be observed numerous glacial striæ. The distance on the street is about 250 feet. From here the dike is lost to view for 130 feet, but may be found in an east- southeast direction upon the adjoining field, where it continues above ground 85 feet. It then again sinks below the surface, but reappears 190 feet further on in the same direction, and there shows an exposure of about 160 feet before finally disappearing. The whole length as thus presented is 815 feet. The width of the trap as it appears above the soil varies from five to twelve feet.
HULL STREET. - Two trap dikes, one three feet wide, the other over four feet, were observed on this street. Their direction was found to be east and west, but irregular.
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The Geology of Hingham.
WEIR RIVER. - In the granite rocks of the east side of Weir River, north of Rockland Street, may be seen several dikes. One may be found a few hundred feet below the Riverside House, ex- tending from the river bank in an east-southeast direction, having a width of six feet. There are two others not far distant having the same general direction, each about two feet wide. Still another was noticed of less width than either mentioned, having pieces of granite, through which it had cut, enclosed.
BEACH NEAR SUMMER STREET. - On the beach east of Hersey's wharf, near the steamboat landing and about ninety yards from it, may be seen a trap dike running east and west, having a width of nine feet. This dike has veins of epidote.
About twenty-eight yards beyond this there is another east and west dike of the same character, which is somewhat irregular and intermixed with granite, but showing, where distinct, a width of two feet.
Fifty yards farther a dike is reached which crosses the beach in the granite, and which is particularly interesting, because it shows within its body a continuous mass of granite which was torn from the walls of that rock and enclosed in the igneous ma .- terial, when this was irrupted from beneath in a molten condi- tion. See Figure No. 2.
FIGURE No. 2.
One hundred and twenty-five feet farther east a small cove, called Mansfield's Cove, is reached, where may be seen just at its
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History of Hingham.
entrance a dike six feet in width, of porphyritic texture and par- tially decomposed, its direction being, like the others, east and west.
The cove is about ninety feet deep, and is bordered on its south- erly side by granite, having here and there more or less mixture of trap. Some Melaphyr is also seen in juxtaposition with the granite, and this rock also appears on the adjoining land near, but to a limited extent.
MARTIN'S LANE. - On the right of Martin's Lane and just beyond its termination, a dike may be observed within granite walls, having an east and west direction and traceable 100 feet. Its width is about six feet.
JOINTS.
Joint structure properly finds place here, as all the rocks of the town exhibit it, and none more than the granites.
Probably there can be found no reader of these pages resident in Hingham who has not observed lines of fracture both in the granitic and the sedimentary rocks of the town, as his eyes have rested upon its numerous ledges. To explain these it will be well to give some account of different kinds of joints that occur in rocks, as they vary in character, have an entirely different origin, and give rise to varied structure ..
The first to occupy attention, then, are such as arise from the contraction by cooling, as in the case of igneous rocks, or by desic- cation, as in the case of sedimentary strata. This contraction results in cracks never parallel or intersecting, and are generally short and not continuous. In some igneous rocks the contraction tends to the formation of polygonal columns, which the joints then surround and embrace. The best exemplification of this structure is seen in the Basalt of the Giants' Causeway in Ireland, where this structure presents the whole rock mass in beautiful prismatic columns, each column separated into blocks having concave and convex surfaces. They vary in dimension and are somewhat irregular, but have been regarded by some as resulting from im- perfect crystallization. There is, however, nothing of crystalliza- tion in their formation, this being without doubt entirely due to contractive action. Professor Crosby has mentioned a case where the columnar structure was observed by him in the felsite of Needham, but no instances of the kind have been noticed in the rocks of Hingham.
The joints next to be mentioned are such as have now re- ceived the name of Joints of Expansion. Almost all rocky masses have, in addition to those of other character, joints, or seams as they are often called, that are approximately horizontal, or nearly parallel with the surface of the ground. They may be observed in any quarry. They divide the rock into layers, and
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The Geology of Hingham.
thus enable the workmen to get out blocks much more easily than would be otherwise possible. The origin of this kind of jointing, as first suggested by Professor Shaler, is now generally admitted by geologists to be due to the effect of the sun's rays upon the surface, leading to a permeation of more or less heat to a con- siderable depth, with consequent expansion, and finally to a sepa- ration of the rock into layers.
The last kind of joints to which attention is called, and the origin of which has been by far the most difficult to explain, are those which are most readily observed upon all the exposed rocks of this town. They may be seen in parallel lines upon their sur- faces, sometimes extending for considerable distances, and often intersected by other lines which are also parallel with each other. These joints are approximately vertical and vary much in direc- tion, which, in view of their probable origin, is an important matter of consideration.
Examination of the direction in many localities shows as follows : -
North and south.
North by west and south by east. North-northwest and south-southeast.
Northwest and southeast.
North-northeast and south-southwest. .
Northeast and southwest.
East and west. East-northeast and west-southwest.
East-southeast and west-northwest.
Others are found varying in direction from all these, but they are not so noticeable.
One of the best localities to observe this joint structure on an extensive scale, although not in this town, will be mentioned here, because it is within a short distance from its boundary and easily observed. It is on Beach Street in Cohasset, very near Sandy Cove, where a large area of rock surface extends from the road- side west on an upward slope, covering a space of several hundred feet. The joints on this surface are particularly well-defined.
The parallel lines under consideration may be observed on almost every exposure of rock, sometimes several feet apart but in other cases only a few inches. At one granite locality on Whiting Street they occur so near each other in some instances as to enable one to pry off pieces not over half an inch thick, specimens of which may be seen in the collection of the Public Library, made to illustrate the geology of the town.
It has always been a source of great astonishment alike to students and casual observers, to find that in the severance of the conglomerate rocks the parts are often found divided as smoothly as if a knife had cut them asunder, and that the very pebbles contained in it are divided with the rest of the mass, instead of
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History of Hingham.
being left intact upon one of the sides of the joint, as would have been judged likely, whatever the force that rent the rock apart.
When two series of the joints under consideration are observa- ble upon any rock surface, those of one series running in a cer- tain direction will be found often to be intersected by those of the other, the result being to separate the rock more or less vertically in rectangular or rhomboidal divisions, and when, as is often the case in slates, there are also joints of expansion, cleavage planes, or planes of stratification, which are more or less transverse to the two mentioned, the rock will break into rectangular or rhomboidal blocks.
Such may be obtained at Huit's Cove or more readily at Slate Island just outside the town limits. Fine specimens may be seen in the town collection of rocks from the former locality.
It remains now to state the probable origin of the vertical in- tersecting joints. Much study has been given to the subject by several geologists. To Professor W. O. Crosby is certainly due the credit of suggesting and ably advocating a theory that seems to the writer after much consideration, conclusive. The theory is that earthquake action caused the phenomena. Space will not here allow further remarks, but the reader who wishes to learn more of it, is referred to the Proceedings of the Boston Society of Natural History, vols. xxii. and xxiii.
THE BOSTON BASIN.
The border line of the granitic and dioritic rocks of the town, whether near or far from the coast and however irregular its course, may be regarded approximately as part of the border of a great area which is known by geologists as the Boston Basin, and which embraces a portion of the towns of Cohasset, Hull, Hing- ham, Weymouth, Quincy, Milton, Hyde Park, Needham, Newton, Brookline, Somerville, Cambridge, Watertown, Malden, Medford, Everett, and the city of Boston, with its harbor east to the outer islands, and possibly a considerable distance beyond, the diameter east-west being not far from twenty-five miles, and having a north- south diameter averaging about twelve miles.
It is absolutely necessary to know much of the history of the formations of the Boston Basin in order to appreciate what may be said of that portion embraced within the limits of Hingham. There has been much discussion carried on over a long period respecting the age and the sequence of its formations. Recent investigations in all parts of it by Professor Crosby have thrown much light upon the subject, changing materially his own views and those of others, who have been informed of his important observations.
One result of his work has been to establish the fact, that in- stead of there being but one formation of slate, as advocated by- himself, there is shown clearly to be two, as claimed by other ob-
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The Geology of Hingham.
servers ; and another is to demonstrate that instead of the sedi- mentary rocks of the basin being of one period, the Primordial, a large portion of them are the deposits of a later age.
Before going further the reader should recognize that in a very early period, probably in Archæan Time, there came to exist over the area of what is now known as the Boston Basin, a great de- pression of the whole surface, probably largely due to subterra- nean igneous action, aided perhaps by long continued erosion by the sea. The certainty that in subsequent ages, through perhaps millions of years, the whole area became as it were a great crater, with violent volcanic action at many periods and in many parts of it, during which vast flows of lava were poured into it, form- ing a considerable portion of its rocks, makes it probable that subterranean action was the chief cause.
Appreciating highly the value of the recent investigations of Professor Crosby referred to above, and agreeing with him gen- erally in his conclusions, the writer believes that he can do no better than to follow him in presenting a summary of the prin- cipal events in the history of the formations within the basin before giving a detailed statement of the sedimentary and asso- ciated rocks of Hingham.
The formation recognized as the oldest in the basin is that of the primordial slates and accompanying Quartzite, known to be of primordial age by the discovery in the slates of Trilobites of that age. These slates occur at Braintree, where only such fos- sils have been found, at Weymouth near by, and in numerous places in the northern portion of the basin. As stated by Pro- fessor Crosby, they probably underlie a large part of the basin covered by the rocks of a later age.
Subsequent to the deposition of the primordial strata a period of violent volcanic action followed, during which were torn asun- der the slates and the quartzite, and vast floods of basic lava, now known as Diorite, were poured in among them and over their surfaces. Following this, there appears to have come a long period of repose and erosion, which was terminated by another of prolonged violent igneous action, bringing to the surface and spreading over it the acid lavas which formed the granite and the petrosilex. As the diorite is found intrusive in the primor- dial strata, and the granite and petrosilex are alike intrusive in the diorite and the primordial strata, it is clear that the latter are the oldest of these, and that the granite and petrosilex are the most recent. If the granites and allied rocks of eastern Massachusetts are, as has been taught by Dr. T. Sterry Hunt and other geologists, Archæan, it may possibly be that these un- derlying the primordial and subjected to intense igneous action, became locally fluent, and thus were injected into and over the superincumbent strata. While, therefore, all thus injected and reformed above the primordial may be regarded as more recent, it may not be true of those outside the basin. There is much,
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History of Hingham.
however, that can be said in favor of the view that all the gran- ites and other rocks of the region, hitherto considered Archaan, are more recent than the Primordial, including even those of the well-known Quincy Hills. Indeed, the evidence that this is the case is well-nigh conclusive. Certainly there can be no question but that considerable areas of the granite were fluent and erup- tive after the primordial slates were formed. A very valuable and instructive article was published in the Proceedings of the Boston Society of Natural History in 1881, by Professor M. E. Wadsworth, on the relation of the Quincy granite to the primor- dial argillite of Braintree, in which he demonstrated that in dif- ferent localities the granite was eruptive through the slates, as shown by the close welding of both rocks, and by the effect of the contact in altering the character of both near the line of junction.
After the events narrated, the area of the basin became one of slow subsidence that must have continued through a vast period of time, as during its ages the great body of the rocks that form the conglomerate series was formed, - the conglomerates and sand- stones near the margins of the coasts, and the slates, the material of which was deposited by the rivers, in the deeper portions. As subsidence continued, the sea encroached more and more upon its shores, the margins of the land became more remote, and the great body of the slate was gradually laid down in the deep waters to a thickness of more than a thousand feet.
Before proceeding further in the history of the basin, the writer will express views long held by him relative to the origin of the pebbles that made up the great body of the conglomerate in- cluding the sandstone, which is only rock of the same character formed of finer material, and of the slates.
Of the conglomerate it may be said that the formation of this rock wherever found has generally been regarded as mainly due to the action of water, and its existence in the Boston Basin has been ascribed to the force of the waves beating for countless gen- erations against, and making an inroad upon, the coast, resulting in the wearing down of the rocks, and the formation by attrition of the bowlders and pebbles which subsequently were cemented into compact strata. This view the writer does not concur in, as he judges it impossible that in any number of ages the action of the waves alone on the area of the basin could have led to the production of such a body of bowlders and pebbles as make up the conglomerate. He believes there was a far more potent cause for their origin silently at work moulding them into form long anterior to their submergence in the surging waters. This cause is to be found in the highly corrosive character of the atmosphere in the early ages of the earth's history, by which the hills, origi- nally of course but rock elevations, became under its action rap- idly disintegrated. Such elevations of early periods in southern regions yet exist as monuments of this corrosive action, for the
.
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The Geology of Hingham.
decayed material remains upon them, showing, though but par- tially, the extent of the corrosion, much of the substance having been washed off the surface by the denuding action of rains.
There is certainly no reason to suppose the general condition of the surface of the land prior to the glacial period was different over the area of the early formations of New England from what prevailed over formations of a like age south of glacial action.
We may therefore picture to ourselves, with good reason, the country everywhere in the neighborhood of Boston covered with hills of considerable altitude, composed of the decayed material of the rocky formations, and having disseminated through it bowlders and pebbles of every size, that had not yet yielded to the decomposing influence. It is well known that corrosive action tends to produce such forms, though of course it is not questioned but that subsequent action of water and attrition had much influence in working a large portion of the pebbles found in the conglomerate into the shapes which they now present.
The subsidence of the area of the basin after the primordial period mentioned, extending the water surface to the base of hills filled with the material for the conglomerate, the igneous action that followed and was active at times during the formation of that rock, causing more or less of oscillation and change of level to the surface, and the subsequent action of the waves upon the cliffs and beaches of coast margin, together, will amply account for the production of the conglomerate, but it will be recognized that the main factor in such view is to be found in the disintegra- tion of the rocky hills long before the action of other forces.
The presentation now made of the origin of the conglomerate of the Boston Basin is greatly strengthened by the fact lately called to the notice of the writer by Professor Crosby, -that no pebbles of the basic rock diorite are found in the conglomerate with those of the acidic rocks. All will agree in the statement that pebbles of the granite, the quartzite, and the petrosilex rocks of the northern border of the basin, have contributed largely to make up the conglomerate ; but what became of those of the diorite, a rock quite as abundant in the ancient hills as any of them ? Its absence can only be accounted for by the view that it could not like the others withstand the corrosive action, as did partially the others, and therefore not even pebbles were left to help form the newer rock. Respecting the slates, their origin is clear. Simultaneously with the depression of the area of the basin below the sea level, there would commence a deposit of the finer sediment brought down by the rivers. This may well be thought to have been copious considering the char- acter of the country passed through, everywhere composed of the decayed remains of the earlier rocks. Indeed it cannot be doubted that the streams would be turbid with argillaceous mat- ter, and, as well known, this would be immediately precipitated upon coming in contact with salt water. Thus the material for
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History of Hingham.
the slates of the basin must have steadily accumulated through. long ages.
The origin of another abundant rock of the basin, associated with the conglomerate, the melaphyr, long continued to be a ques- tion of much discussion, but there is now no doubt concerning it. During all the immense time that subsidence continued, and while sedimentary strata were gradually accumulating, the area of the basin remained a great centre of igneous action, and vol- canoes here and there within it belched forth from time to time floods of lava which spread itself over the surface. Professor Crosby has made out in the Nantasket region several flows of it, each of which alternates with deposits of conglomerate and sand- stone. In such cases the outpouring was probably beneath the surface of the water, where the deposits followed each period of activity. In Hingham the melaphyr is found in very great bodies not separated by deposits of the sedimentary rocks.
One more great event in the history of the basin is yet to be mentioned. Long after the volcanic action that had produced the basic lava, melaphyr, had ceased, and after all the sediments were deposited that produced the rocks known to us as the conglomer- ates, the sandstones, and the slates, a great disturbance occurred over the whole area of the basin and of the crystalline rocks surrounding it, caused by another manifestation of igneous en- ergy, which changed the whole character of the surface. With- in the basin, apparently from immense pressure exerted in north and south directions, the rocky strata were forced up in folds or in broken ridges. Through crystalline rocks and sedimentary strata alike, subterranean action brought to the surface, and probably poured over it, vast quantities of lava of highly basic. properties, different from those of the previous eruptions, now known to lithologists as Diabase, an account of which has been given. The great erosion of after ages is undoubtedly the rea- son why the rock Diabase is not found spread over the surface, as well as within the walls of dikes.
At length the disturbing action ceased, and the earth, which had been shaken from its foundations to its surface, and rent asunder in a thousand localities, once more became quiescent. The effect upon the area of the basin was great, for where the waters had for an immense period spread themselves over the surface, and under which conglomerates and slates had been laid down, dry land appeared.
How strange to reflect that in these three words is embraced a fact without which all the stupendous events that have been mentioned, occurring over millions of years, would have re- mained entirely unknown to mortal man; for with the waters covering the basin, where could a trace of its long history have been found ?
The rocks of the Boston Basin as they present themselves in Hingham will now be noticed. Unfortunately the non-occurrence
42
History of Hingham.
the slates of the basin must have steadily accumulated through long ages.
The origin of another abundant rock of the basin, associated with the conglomerate, the melaphyr, long continued to be a ques- tion of much discussion, but there is now no doubt concerning it. During all the immense time that subsidence continued, and while sedimentary strata were gradually accumulating, the area of the basin remained a great centre of igneous action, and vol- canoes here and there within it belched forth from time to time floods of lava which spread itself over the surface. Professor Crosby has made out in the Nantasket region several flows of it, each of which alternates with deposits of conglomerate and sand- stone. In such cases the outpouring was probably beneath the surface of the water, where the deposits followed each period of activity. In Hingham the melaphyr is found in very great bodies not separated by deposits of the sedimentary rocks.
One more great event in the history of the basin is yet to be mentioned. Long after the volcanic action that had produced the basic lava, melaphyr, had ceased, and after all the sediments were deposited that produced the rocks known to us as the conglomer- ates, the sandstones, and the slates, a great disturbance occurred over the whole area of the basin and of the crystalline rocks. surrounding it, caused by another manifestation of igneous en- ergy, which changed the whole character of the surface. With- in the basin, apparently from immense pressure exerted in north and south directions, the rocky strata were forced up in folds or in broken ridges. Through crystalline rocks and sedimentary strata alike, subterranean action brought to the surface, and probably poured over it, vast quantities of lava of highly basic properties, different from those of the previous eruptions, now known to lithologists as Diabase, an account of which has been given. The great erosion of after ages is undoubtedly the rea- son why the rock Diabase is not found spread over the surface, as well as within the walls of dikes.
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