L. V. Pirsson — Geology of Conanicut Island. 371 



Dikes. — The granite is cut through by aplite dikes in seve- 

 ral places, an excellent example being a small one of but a few 

 inches in width which cuts the exposed rock below and to the 

 east of Mr. Clothier's house on Bull's Point, and which has 

 been bent and faulted by later movements. Besides these 

 which are in intimate connection with the granite there are 

 two other dikes on the island, both on the Beavertail portion 

 and cutting it and the strike of the beds transversely; see fig. 

 1. They are not large, the one at Hull's Cove is the larger 

 and at this exposure from eight to ten feet wide. Its outcrop 

 can be traced for some distance in a series of nearly buried 

 bowlders. 



The second one, a little south of the point known as Lion's 

 Head is still narrower. Its western outcrop occurs on the 

 south point of Austin's Hollow. 



These dikes are of the lamprophyre rock known as minette 

 or mica trap as will be shown later. They stand in close 

 petrological relation to the granite and must be considered as 

 of contemporaneous origin. 



They have altered the adjacent shales, the one at Hull's Cove 

 in particular having produced a rock similar in appearance to 

 some of those formed in the contact zone of the granite as 

 previously described 



Orographic movements. — Besides the original folding of the 

 Carboniferous series the island has suffered great dynamic 

 pressures subsequent to the intrusion of the igneous rocks. 

 The thrust has been in a general north and south direction. 

 The granite has been crushed and dynamo-metamorphosed in 

 places ; planes of fracture extend through it generally in an 

 E. and W. direction, often highly slickensided as in an excel- 

 lent exposure at the gate to the grounds of Mr. J. Wharton in 

 C 3. The aplite dikes in it have been bent and broken. The 

 minette dikes on Beavertail have also been bent and folded 

 and highly pressed ; in places they have become quite schis- 

 tose as at the south point of Austin's Hollow and the schistosity 

 is perpendicular to the thrust. The sedimentaries have also 

 suffered from these forces, steps in the shales running E. and 

 W. being of frequent occurrence and the beds in other places 

 have been fractured and bowed up as at X, the fractures and 

 hollows from bending being now filled with veins and lenticu- 

 lar masses of secondary quartz. 



General summary. — From the foregoing facts the following 

 general statement may be deduced for the geology of Conani- 

 cut Island. That it is in the main made up of a series of Car- 

 boniferous strata thinly bedded with a conglomerate as the 

 oldest member, which has been folded so that the beds have a 

 general north and south strike and dip mostly to the eastward. 



