CHAPTER III. 



THE IGNEOUS ROCKS OF THE BORDER OF THE BASIN. 



GRANITIC ROCKS. 



The granitic rocks which border this basin from Wrentham eastward 

 and thence along the margin in Plymouth County southward and westward 

 to Fall River are designated Archean on the most recent maps. The rocks 

 are mainly hornblendic granitites. 1 The opinions which have been published 

 regarding the origin and time of eruption of these rocks are various and 

 often contradictory. Edward Hitchcock, if he can be said to have held a 

 definite opinion upon the subject, was inclined to represent the granitic 

 masses as erupted after the Carboniferous formations were deposited. In 

 1882 Wadsworth clearly showed that the hornblendic granitites in Braintree 

 are erupted through slates now known to be of middle Cambrian age. The 

 granitite of that locality is then more recent than the middle Cambrian. 

 Except for a few detached areas, all around the border of this part of the 

 basin the granitite extends beneath the Carboniferous basal beds, which are 

 in large jmrt made up of the little-worn waste from the granitite. At no 

 point in the northern half of the field has true granite been found in 

 eruptive contact with Carboniferous sediments. 



The coarsely crystalline texture of the granitite along most of the 

 margin indicates, according to the accepted opinion, that the rock which is 

 now exposed at surface crystallized at a depth and under a cover of some 

 thickness, probably in part of sedimentary rocks removed from this portion 

 of the area before Carboniferous deposition set in. Some of the Cambrian 

 formations undoubtedly formed part of this cover. No small depth of the 

 granitite must likewise have been carried away. At Braintree the zone of 

 granitite with a finer texture, due to the more rapid cooling near contact 

 with the slates, is preserved, and gives a minimum thickness of several 

 scores of feet between the contact and the inner coarsely crystalline rock. 



Although the several varieties of granitic rocks in the area named are 



1 The " syenite " of Edward Hitchcock and earlier writers. 



