94 REVIEWS 
with a short table for determining rocks. The illustrations are numerous 
and remarkably well chosen. 
In discussing the origin of the porphyritic texture in igneous rocks 
Pirsson expresses the current view among petrographers in declaring that 
the idea of a change in the rate of solidification (for example, intratelluric 
and extratelluric crystallization) is not an adequate explanation of all occur- 
rences. But it seems to the writer that the labile and metastable states 
proposed by Miers and indorsed by Pirsson as an explanation of the por- 
phyritic texture are equally unsatisfactory. It seems to merely give names 
to certain conditions or states in magmas which may produce the texture, 
without explaining anything. Why such conditions should exist in some 
cases and not in others—in fact, why they should exist at all—is not clear. 
The writer would suggest that if we admit the existence of eutectics in igne- 
ous rocks (and Pirsson appeals to them to explain salic border zones) we 
have in their laws a reasonable explanation of the porphyritic texture. 
Thus, it is well known that any constituent present in a solution in greater 
amount than the eutectic proportion will begin to crystallize at a tempera- 
ture above that required for the solidification of the eutectic itself, and will 
continue to crystallize until the cooling reaches that temperature; then the 
eutectic will crystallize at that temperature. It is clear then that with a 
uniform rate of cooling this process will give a much longer period of crystal- 
lization to the minerals in excess of the eutectic proportion than to the 
eutectic itself. This longer period of crystallization would naturally result 
commonly in larger crystals, that is, the porphyritic texture. In this con- 
nection it might be mentioned that Pirsson’s statement on p. 171 that “‘the 
substance in greatest excess, the solvent, will solidify first” is quite mislead- 
ing, since the substance in greatest excess is not necessarily the solvent nor 
the first thing to solidify. 
Other minor errors include the crediting of tests of Wisconsin granites 
(p. 209) to Bain instead of to Buckley, and the omission of silicon in the 
paragraph on the elements of geological importance (p.19). On p. 135 
Pirsson defines as “‘hade” and ‘‘trend” what are ordinarily called dip and 
strike. It is not clear that anything is gained by the change, and it must 
result in some confusion. On the other hand he draws the distinction (p. 
158) sharply and well between textures and structures in rocks, and describes 
numerous examples of each. His discussions of the difficult subjects of 
metamorphism, differentiation, etc., are remarkably well adapted in their 
simplicity and clearness to the place they occupy. As is to be expected, 
they reflect chiefly the views of the German school of petrographers. 
A. N. W. 
