The Geology of H Ingham. 39 



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 Archaean Time, there came to exist over 

 the area of what is now known as the Boston Basin, a groat 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 pctrosilex. 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 pctrosilex 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, Archaean, 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 primordinl may be regarded as more recent, 

 it may not be true of those outside the basin. There is much, 



