2 14 REVIEWS 



author proposes to designate "all acid volcanic rocks the structures of 

 which proves them to have once been glassy, aporhyolites, while such 

 as were originally holocrystalline, or whose original character is in 

 doubt, will be termed quartz-porphyries." The prefix apo is intended 

 to indicate the fact of a special kind of alteration, namely, that of 

 devitrification of a solid glass. Its application is therefore limited to 

 a certain class of rhyolites, that is, the hyalorhyolites. 



The varieties called quartz- porphyry are briefly described, and call 

 for no special comment, except to note the occurrence in them of the 

 unusual mineral piedmontite together with ordinary epidote, both 

 being secondary minerals. The aporhyolites are described at length. 

 They are characterized by numerous spherulites and some lithophysae, 

 most clearly reorganized on weathered surfaces of the rock. Their 

 form and distribution are the same as in recent, unaltered obsidians. 

 The description of the phenocrysts of feldspar is not entirely satisfac- 

 tory and their actual character is left in doubt, except that they are 

 undoubtedly alkaline varieties. 



The microscopical study of the groundmass has been very thor- 

 NDUghly carried on, and the descriptions make it evident that there 

 once existed in these rocks textures commonly found in modern rhj'o- 

 lites, such as flow-structure, taxitic structure, perlitic cracking and 

 spherulitic crystallization, making it highly probable that these rocks 

 originally solidified in a partially glassy condition. They are at pres- 

 ent holocrystalline and exhibit a microcrystalline and also a micro- 

 poikilitic texture. The latter is discussed at great length and its- 

 secondary character in these rocks is clearly established. The author 

 recognizes the fact that the sanie or a similar texture is also a primary 

 crystallization in certain other rocks. Flow-breccias and tuft-breccias 

 are found in connection with the massive lavas. In places the massive 

 rock is metamorphosed into a sericite-schist, in which often the original 

 phenocrysts are still preserved. The chemical conipositions of the two 

 rocks are nearly identical. 



The petrographical description of the basic eruptives begins with 

 a discussion of the nomenclature relative to these rocks, which is short 

 and leaves the subject in a confused condition; the confusion being 

 carried throughout the chapter. The confusion i?^ based on the con- 

 clusions arrived at in the paper by Professor W. S. Bayley, which is 

 quoted by the author. The mistake is made in assuming that the 

 definition of the groups within the gabbro class of rocks as suggested 



