CLASSIFICATION OF IGNEOUS ROCKS 627 



in a rock by using the name of the mineral without modification, 

 compounding it with the magmatic name by means of a hyphen. 



In case there are several critical minerals present, their 

 names are to be used in such order that the most abundant stands 

 nearest to the magmatic name, and the least abundant the 

 farthest from it. 



It may become desirable when the number of mineral names 

 is considerable to abbreviate and compound them in the manner 

 suggested by Chamberlin, 1 and Jevons. 2 



It is to be remarked that it will not always be necessary to 

 state the exact character or species of the critical mineral. It 

 will often merely be necessary to mention the mineral group, 

 whether augite, hornblende, mica, garnet, etc. This is because 

 experience shows that in a magma of a given character the 

 augite or hornblende formed will in general be of a more or less 

 constant character. Thus in highly calcic magmas the augite 

 will in general be of one kind, in sodic magmas of another, and 

 in magnesic magmas of another. 



The critical minerals, especially the alferric, may be devel- 

 oped within wide limits, which in the more femic Classes may be 

 illustrated by three rocks of the dosodic Subrang of the alkali- 

 calcic Rang of the Order portugare, of the salfemane Class. In 

 this we find the hornblendite of Gran, which consists entirely of 

 an alkalic hornblende, with no visible salic or femic minerals. 

 Here also belongs the olivine-gabbro-diabase of the same locality, 

 in which much of the feldspar has actually crystallized as such, 

 and in which the hornblende consequently is less alkalic, and 

 augite is present. We also find here a nephelite-basanite of 

 Colfax county, New Mexico, in which the actual composition 

 corresponds quite closely with the norm. The chemical analyses 

 of these three rocks show their chemical likeness. 



In the first case all the possible alferric mineral has been 

 formed, as hornblende, in the second case only a part, approxi- 



1 Geology of Wisconsin, Vol.1, pp. 30-40. Madison, 1883. 



2 Geo/. Mag., Vol. VIII, p. 304, 1901. 



