840 THE JOURNAL OF GEOLOGY. 



In South America the great Cordilleran system of the Andes 

 presents a petrographical province identical, chemically and 

 mineralogically, with those of the North American Cordilleras, and 

 which appears to extend throughout its entire length. In the 

 eastern part of the continent and on the islands off its coast the 

 petrographical province is in turn identical in many respects with 

 the eastern province of North America ; the correspondence being 

 most pronounced between the rocks from Brazil, described by 

 Derby 1 , and those from Arkansas described by J. Francis Wil- 

 liams. 2 



The chemical and mineralogical qualities or peculiarities which 

 characterize the rocks of particular groups, and at the same 

 time serve to distinguish them from those of some other groups, 

 are like family traits of character, and suggest the intimate rela- 

 tionship and common origin of all of the igneous rocks of the 

 group. They prove conclusively that the varieties of rocks 

 occurring at a particular center of eruption, or in a volcanic dis- 

 trict, have been derived from some magma common to the dis- 

 trict by a process of differentiation similar to that which has 

 caused smaller bodies of molten magma to become chemically 

 heterogeneous and has produced mineralogical facies. 



That the process which has produced the many kinds of igne- 

 ous rocks in any region, with all their transitions into one 

 another, was a process of differentiation of an originally homo- 

 geneous magma, and not the compounding of two or more dif- 

 ferent ones, is shown by the geological relationships between the 

 various bodies of rock belonging to a volcanic center ; more 

 especially the order in which they have been erupted. A process 

 dependent upon any set of physical conditions, which continues 

 active for long periods of time must yield results that are to a 

 very considerable extent functions of time, that is, they must be 



1 0. A. Derby : On Nepheline Rocks in Brazil, with special reference to the Associa- 

 tion of Phonolite and Foyaite. Quart. Jour. Geol. Soc. 8vo, London, Aug., 1887. 

 Also The Tingua Mass. Ibid., May, 1891. 



2 J. Francis Williams : The Igneous Rocks of Arkansas. Annual Report of the 

 Geological Survey of Arkansas for 1890. Little Rock, 1891. 



