53° 



PETROLOGICAL ABSTRACTS AND REVIEWS 



new title was used. The book is designed for the working geologist while 

 "the more elementary portions are chiefly intended for the student"! 

 As has been the case with many recently issued petrological textbooks, 

 the first portion is given up to the rock-forming minerals, 80 pages being 

 here so employed. Chapters are devoted to the classification, structure, 

 origin, metamorphism, and weathering of rocks, and various rock types 

 are discussed. The author bases his classification of the igneous rocks 

 upon their chemical composition, and divides them, in a very simple 

 manner, by their silica content, thus: 



This classification is simple enough, but it does not show genetic 

 relationships. It would be impossible, by the use of the microscope, to 

 classify rocks according to this system. Chemical analyses would have 

 to be made of every rock, a prerequisite entirely out of the question in 

 practical work. Furthermore, the names thus limited by the silica con- 

 tent have already been used for rocks of certain mineralogical composi- 

 tion, and the new definitions for the old terms cause further confusion in 

 an already almost hopelessly confused nomenclature. 



Albert Johannsen 



Merrill, George P. ''On the Supposed Origin of the Moldavites 



and Like Sporadic Glasses from Various Sources," Proc. U.S. 



National Museum, XL (19 11), 481-86. 



The greenish, chrysolite-like glass pebbles found in many regions and 



called moldavite, billitonite, australite, obsidianite, or obsidian bombs, 



are all included by Suess under the name tektites, and are regarded by 



him as being of ultra-terrestrial origin, the markings being a consequence 



of their mode of origin. To this Merrill takes exception, regarding the 



Bohemian and Moravian specimens as water-worn pebbles of weathered 



glass, originally etched by corroding vapors or solutions, and the 



Australian forms as pebbles, water-worn or abraded by wind-blown 



sands. He does not wish to controvert the theory of an original cosmic 



