14 SCIENCE PROGRESS. 



his study of the Tertiary igneous rocks of the Inner Heb- 

 rides, and similar conclusions have been arrived at by Lotti 

 in Elba and by various other geologists in the Andes and 

 elsewhere. 0( special importance in this connection are the 

 researches of Hague and Iddings on the rocks of the Com- 

 stock Lode in Nevada. The extensive mining operations 

 at that place enabled them to make a comparative study of 

 the rocks through a very considerable vertical range, and 

 they conclusively established the gradual transition in cha- 

 racters from andesitic rocks consolidated under practically 

 superficial conditions to the plutonic equivalents of those 

 rocks formed at greater depths and in the heart of the mass. 

 Of interest, too, is the close correspondence, shown by 

 Iddings (i) in the Yellowstone Park district, between the 

 intrusive porphyrites and diorites of Electric Peak and 

 the breccias and lavas of Sepulchre Mountain. Speaking 

 generally, the geological evidence warrants us in affirming 

 that igneous intrusions have occurred in districts of volcanic 

 outbursts, and are so related in point of time to the latter 

 as to indicate an essential connection between the two 

 phenomena. 



The growth of opinion concerning the petrology of 

 igneous rocks has been, in recent years, mainly around two 

 central ideas, vis., the close relation subsisting between 

 igneous activity and movements of the earth's crust, and the 

 production under proper conditions of various rock-types 

 from one oritrinal rock-masfma. Neither of these ideas is 

 strictly a new one. The former was insisted upon long ago 

 by von Buch and others ; but while those writers regarded 

 volcanic outbursts as the causes of disturbances of the strata, 

 modern geologists take rather the converse view. Again, 

 the conception of differentiation in molten rock-magmas is 

 at least as old as Durocher's famous theory ; but only 

 recently has the idea been developed with any attempt at 

 completeness, and the practical application of it to a par- 

 ticular group of associated rocks had scarcely been ventured 

 upon before Brogger's studies in the Christiania basin. 



It is with the former of the two ideas just mentioned 

 that we are here concerned. The work of Suess, Lossen, 



