116 A. Wandke — Intrusive Rocks of 



Rye formation. The effusives, or at least rocks resem- 

 bling metamorphosed effusives, occur in a narrow belt 

 at Portsmouth, New Hampshire. No attempt was made 

 to study them. The discussion will center upon the dikes 

 and subjacent bodies which for the most part cut the 

 Upper Carboniferous rocks. 



Dikes. 



The dikes, because of their number, their wide range 

 in composition, and the manner and order of their 

 intrusion form a group of rocks well deserving of more 

 detailed study. Although every outcrop carries one or 

 more of them, the best display of dikes occurs along the 

 coastal section from Perkin's Cove to Brave Boat 

 Harbor. In these eleven miles there are several hundred, 

 perhaps a thousand, dikes and they vary in width from 

 stringers hardly thicker than the blade of a case knife 

 up to 200 feet. Multiple and composite examples 

 abound. In several instances three or more dikes of 

 the same or of contrasted compositions may occupy a 

 single fissure. In composition they range from an 

 olivine-bearing lamprophyre through diabase, diorite, 

 and granite porphyry to paisanite and aplite. Some 

 ^abound in inclusions, which being fragments either 

 brought up from depth or torn from the adjacent walls, 

 occur in various stages of assimilation. In short, the 

 dikes of the area form a remarkable display and illustrate 

 most of the phenomena associated with intrusions of 

 this type. All of the observed dikes are later than both 

 the period of folding and the period of granitization 

 which produced the Rye gneisses, and for the most part 

 appear younger than the quartz veins and the metamor- 

 phism which characterize the outcrops from Perkin's 

 Cove to Brave Boat Harbor. In a broad way they may 

 be separated into two groups, 12 (a) those earlier, and (b), 

 those later than the stocks and batholiths. On the basis 

 of composition each of these groups may be subdivided 

 and such a sub-division is helpful in gaining a clearer 

 conception of the changes in the magma from which the 

 dikes were derived. 



12 A similar grouping was made by C. H. Clapp for the dike rocks of 

 Essex County, Mass., U. S. Geol. Surv., Bull. 704, p. 107, 1921. 



