ALG ONKIA N RO CKS IN VERMONT. 4 1 9 



it with probability to an original diabase.^ Remains of an orig- 

 inal bisilicate (augite) can still be found in the rocks. Whether 

 diabase or basalt their occurrence in sheets traversed by dikes 

 of the same material and their great abundance lead me to con- 

 sider them surface flows or intrusives. Their abundance may be 

 cited as evidence of extrusive origin since it is extremely unlikely 

 that any area, reasoning from analogy, would be traversed by so 

 large a number of intrusives. This view is also sustained by the 

 fact that diabases and basalts are prevailing surface flows. Such 

 regions as the Triassic (Newark) of the eastern United States, Kew- 

 eenaw Point, the western plateau, and the Deccan being examples. 

 Their restriction to the Mount Holl}^ series not only points to 

 their extrusive origin, but whatever their origin they afford 

 almost positive evidence of an unconformity at the top of the 

 series ; if intrusive, we should naturally expect to find them 

 occurring in the Mendon series, which is not the case ; if extru- 

 sive, their occurrence only in the core rocks is even more in favor of 

 the proposed subdivision. As to the importance of the evidence 

 afforded by these rocks no better confirmation can be found than 

 the following from Van Hise.^ " Eruptive rocks are often an 

 important guide in determining structural discordances. These 

 are valuable when the older series has passed through an epoch 

 of eruptive activity before the newer series was deposited. In 

 such cases, bosses, contemporaneous or intrusive beds, volcanic 

 fragmental material or dikes may occur in the older series which 

 nowhere are associated with the newer. It is possible, of course, 

 that eruptives may penetrate the inferior members of a series and 

 never reach the higher formations ; but if it is found that the 

 supposed inferior series is associated with abundant material of 

 igneous origin which never passes beyond a certain line, it is 

 almost demonstrative evidence of the later age of the newer 

 series." 



' Geology of the Green Mountains in Massaciiusetts, by R. Pumpelly, J. E. Wolff, 

 T. Nelson Dale, and Bayard T. Putnam, Monograph U. S. Geol. Survey, Part 3, sub- 

 mitted in 1889. 



' Correlation Papers — Archaean and Algonkian, Bull. No; 86, U. S. Geol. Survey, 

 p. 520. 



