The American Naturalist. 
MINERALOGY AND PETROGRAPHY.' 
Petrographical News.— An epitome of the various facts known 
to petrographers with relation to the act of crystallization in rock 
magmas, and the conditions under which this takes place, has been 
sorely needed by students who are unable to keep abreast of the wide- 
spread literature of modern lithology. Mr. Iddings' has recently 
succeeded in presenting the subject to us in a manner that is at the 
same time scientific and untechnical. His work will surely be appre- 
ciated by all of his co-laborers in petrography, as well as by students 
and geologists at large. Mr. Iddings' paper is divided into two 
distinct parts. In the first the phenomena of crystallization are dis- 
cussed, with especial reference to the crystallizations of mineral in rock 
magmas. The second portion of the article deals with the causes of 
crystallization ; and in this it is that the author has given the most 
valuable results. After mentioning the cases in which rocks and rock- 
forming minerals have been artificially produced, and calling attention 
to the analogy that exists between the originally molten rock-magma 
and saturated solutions of salts, it is concluded that (i) the order of 
crystallization in rocks depends primarily upon their chemical compo- 
sition, and (2) its nature upon the physical conditions obtaining 
during the solidification ; the principal physical conditions affecting 
crystallization, in the order of their importance, being temperature, 
rate of cooling, chemical composition of the original magma, the 
presence of mineralizing agents (aiding in the formation of crystals, 
e. g., water in many rocks), and finally pressure. The effect of each 
one of these conditions is briefly alluded to, and the impression which 
each leaves upon the cooling magma during its progress from the 
interior of the earth to its surface is clearly described. In the course 
of the article the word phenocrysts is suggested as an equivalent for 
the German word " Einspringlinge,"— porphyritic crystals. The 
controversy between Prof. Judd and Dr. Geikie as to the origin of the 
massive rocks of the Western Isles of Scotland has been intensified by 
the appearance of Geikie's' " History of Volcanic Action During the 
Tertiary Period in the British Isles," and a reply to this by Prof. 
eCryst 
