GENETIC RELA TIONSHIPS AMONG IGNEOUS ROCKS. 839 



closely allied to the chemically nearest variety in their own 

 group than they are to the rock of the same name in another 

 group. 



It must not be inferred from the facts just given that every 

 natural group of rocks has some peculiarity which distinguishes 

 it from every other group. There are many natural groups or 

 petrographical provinces, the rocks of which are identical in the 

 minutest detail with those of neighboring or distant regions. 

 And the limits or boundaries of such provinces are not sharply 

 drawn in nature. In some regions the transition from one prov- 

 ince to another appears abrupt, in others very gradual. Thus, 

 while certain provinces exhibit distinct mineral and chemical 

 characteristics, others appear to possess characters of several 

 provinces. 



Recognizable chemical differences may exist between groups 

 of rocks within less than a hundred miles of one another, and 

 again broad general features may be persistent, or at least may 

 be prevalent, over vast areas of the globe. Within these areas, 

 of course, subordinate variations may exist. The most impres- 

 sive illustration of this law is furnished by the igneous rocks of 

 the two continents of North and South America. The great belt of 

 Cordilleras and parallel ranges stretching along the western side 

 of North America abound in igneous and volcanic rocks which 

 belong to a quite uniform petrographical province, extending from 

 British Columbia to Mexico and Central America. They are not 

 specially rich in alkalies, and are characterized by a very general 

 presence of the ferro-magnesia mineral, hypersthene ; local varia- 

 tions occur. As the eastern portion of this mountain system is 

 approached from the west a gradual increase in alkalies is notice- 

 able, and rocks bearing nepheline, leucite and more frequent 

 alkali-feldspars make their appearance, containing alkali-bearing 

 ferro-magnesian minerals. These have already been described, 

 from Montana, Wyoming, Dakota, Colorado and Texas, and are 

 especially well developed in Arkansas. Similar eruptive rocks 

 have been found in the eastern portion of the continent, in New 

 Jersey, New England and Canada. 



