﻿W. S. Shaler — Geology of Cobscook Bay District, Me. 43 



which have taken place. These dykes are most extensively 

 developed in the northern part of the Cobscook district, es- 

 pecially in the area about McMaster's Island and in the region 

 to the westward as far as Deny's Eiver. The felsites are, as is 

 well known, found in the region about Lynn in Massachusetts. 

 The Cobscook felsites do not however exhibit the same dis- 

 tinct flow lines as those from the more southern localities. 

 They have not been observed in the clearly banded or ribboned 

 form which they often exhibit in the region near Boston. 



Besides these extensive extrusions of lavas belonging to the 

 group of felsite porphyries, there is a larger series of other 

 dyke stones and sheet lavas which exhibit a very great variety 

 of appearance. Some of them are of a blackish, slaggy aspect, 

 showing a tolerably distinct bedding, which seems to arise 

 from successive flows. The best exhibition of these slaggy 

 lavas is at the cape known locally as Denbo Point, the extrem- 

 ity of which is formed by them. 







Hightide 



-J**'**" 









i4§? 



Xmttdt 



C 



8 



^?^5SS:S|S 



\\ miles south of Denbo Point. 

 A, greenish mottled trap. B, metamorphosed conglomerate. C, greenish amyg- 

 daloid al trap. 



Besides the above mentioned lavas there are many other 

 varieties of igneous rocks found in this district which are 

 not classifiable by ordinary microscopic characters. These 

 resemble in their general aspect the ordinary dykes of eastern 

 New England. The classification of all these volcanic pro- 

 ducts must be left until they can be submitted to a careful 

 lithological study. 



The rocks of Silurian age are so cut up by these various 

 intrusions that not more than one-third of the whole land area 

 intersected by the arms of Cobscook Bay, is occupied by these 

 strata. It seems likely that the sedimentary Paleozoic strata 

 are extensively developed beneath the surface of the bays, the 

 intervening ridges of land being occupied in the main by the 

 harder volcanic rocks. That this is the case, is well shown by 

 the fact that the stratified Paleozoic rocks are generally found 

 as a fringe along the shores of the long, narrow capes which 

 divide this bay into many inlets. These Paleozoic rocks are 



