Page:The Journal of geology (IA journalofgeology11893univers).pdf/423
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 province 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 impressive 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 variations occur. As the eastern portion of this mountain system is approached from the west a gradual increase in alkalies is noticeable, 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.