834 THE JOURNAL OF GEOLOGY. 



viscous, and crystallization generally takes place, but the two are 

 in a measure independent operations, and the viscosity may be 

 advanced so rapidly that crystallization is more or less completely 

 prevented and glassy rocks result. According to the conditions 

 under which rock magmas cool solidification will be accompanied 

 by more or less complete crystallization. The size also of the 

 crystals will vary with the rate of cooling, and the general 

 texture of the rock will be affected. Different parts of one rock 

 magma may experience different conditions of cooling, and there 

 will result a variety of textures or structures within the mass. 

 It may be that the textural differences are sufficiently pronounced 

 to be given distinctive names, which become the terms by which 

 certain kinds of rocks are designated ; for example, granite, 

 porphyry, pearlite, pumice, etc. There is then a relationship 

 between certain kinds of igneous rocks which exists because of 

 different conditions which have attended the solidification of 

 various portions of one body of magma, or of several magmas 

 alike in other respects. The significance of this relationship was 

 long ago appreciated by James D. Dana, 1 who maintained that 

 the textural differences among rocks were mainly due to the 

 physical conditions under which they consolidated ; an idea ably 

 advocated and corroborated by Judd, 2 and more recently sub- 

 stantiated by numerous observations in many localities. 



Igneous rocks often differ from one another in mineral and 

 chemical composition ; in fact, some kinds differ so widely from 

 one another in a mineralogical sense that they possess no mineral 

 in common. And most kinds contain the minerals which may 

 be common to them in quite diverse proportions, and associated 

 with various other species. Chemically they consist of the same 

 essential constituents in variable proportions, the variations being 

 within certain limits. But the proportions are so far from being 



'United States Exploring Expedition during the years 1 838-1 842, under the com- 

 mand of Charles Wilkes, U. S. N., 410. Philadelphia, 1849, Vol. 10, Geology, p. 

 372 et seq. 



2 J- W. Judd: On the Ancient Volcano of the District of Schemnitz, Hungary. 

 Quart. Jour. Geol. Soc, 8vo, Vol. 32, 1876, p. 292 et seq. 



