REVIEWS 213 



The Ancient Wilcanic Rocks of Sotitli Mojintaiii, Pennsylvaitia. 

 By Flokknck Bascom. Bulletin U. S. Geological Survey, 

 136, Washington, 1896. 



When the work on the })orphyries of South Mountain was taken up, 

 little was definitely known regarding the occurrence of ancient volcanic 

 rocks in the eastern United States outside of the region in Massachu- 

 setts in which certain felsites had been found. Having come across a 

 specimen of the porphyry from South Mountain, Professor G. H. 

 A\'illiams and Miss Bascom visited the region to learn more with regard 

 to its occurrence, and finding unmistakable evidence of the presence of 

 an ancient igneous rock with pronounced flow structure and indications 

 of spherulitic crystallization, it was decided to make a special inves- 

 tigation of the region. This was carried on in the summer of 1892, 

 Professor Williams studying the northern part of the region and Miss 

 Bascom the southern part. The latter also undertook a detailed study 

 of the igneous rocks there found. Preliminary notices of the general 

 geology of these volcanic rocks have been published by Professor 

 W'illiams, and of the petrography of the most siliceous varieties by Miss 

 Bascom. The present publication presents the complete investigation 

 of all the igneous rocks of the region. 



It reviews the literature bearing upon the district from 1755 to 

 1S96. From this it is clear that the true character of the more sili- 

 ceous rocks was not understood by previous investigators. Three 

 tvpes of rock occur, one a sandstone, conglomerate and quartzite, with 

 occasional argillaceous shale ; another an acid volcanic rock, and the 

 third a basic volcanic rock. The sandstone is referred to the Lower 

 Cambrian Age, though not definitely; and the igneous rocks are found 

 to be older than the sandstone, being extrusive lavas overlaid uncon- 

 formably by the sandstone. They are considered to be of pre-Cam- 

 rian Age, and their petrographical resemblance to the Keweenawan 

 volcanic rocks of the Lake Superior region is pointed out. The acid 

 rocks are probably older than the basic ones. But this could not be 

 definitely determined. 



Before entering upon the petrographical description of the acid 

 eruptive rocks the author finds it necessary to devote four pages to a 

 discussion of the nomenclature of the aphanitic, porphyritic and non- 

 jiorphvritic varieties of these rocks — an excellent commentary on the 

 ])resent condition of i)etrogra])hic terminology. At its conclusion the 



