304 THE EVOLUTION OF PETROLOGICAL IDEAS. 



petrographers. Thus Professor Iddings, in the introduction to his 

 important memoir on the Origin of Igneous Rocks, says: 



''The object of the present paper is to give the writer's reasons for 

 concluding that all of the volcanic and other igneous rocks of any 

 region are so intimately connected together by mineralogical and 

 chemical relations that they must have originated from some single 

 magma Avhose composition may be different in different regions; and, 

 further, that it is the chemical differentiation of this primary magma 

 which has given rise to the various kinds of igneous rocks." 



The fact that the diverse igneous rocks of certain districts are often 

 bound together by common mineralogical and chemical characters 

 which distinguish them from the corresponding rocks of certain other 

 districts was clearly recognized by Professor Judd in his well-known 

 paper on the Volcano of Chemnitz, and subsequently crystallized by 

 him in the happy expression petrographical province, as applied to 

 any district in which the igneous rocks have certain common charac- 

 teristics. The idea has been still further extended and ela])orated T)y 

 Professor Iddings, who sees in the conunon characteristics the indica- 

 tions of a kind of blood relationship or consanguinity, which can onlv 

 be explained on the assumption that the different species of one and 

 the same province have originated by differentiation from a single 

 homogeneous magma. 



Professor Brogger, in his remarkal)le series of studies on the rocks 

 of the Christiania district, has still further generalized this idea, and 

 much of his work is directed toward the evolution of a genealogical 

 tree, in which the twigs shall correspond to the final products of dif- 

 ferentiation, the larger branches to some of the plutonic masses, and 

 the trunk to the primordial homogeneous inagma. The idea is a fas- 

 cinating one: se non e vero, e ben trovato. But it must be admitted 

 that we know very little about the causes of the assumed differentia- 

 tion. These are supposed to be of two types: (1) those which affect 

 the licpiid magmas, and (2) those connected with the separation of the 

 minerals. Magmatic differentiation is generally regarded as the most 

 important, but it is the type of which we know least. Soret's princi- 

 ple, to which I have appealed, will, I fear, help us veiy little, though 

 it is undoubtedly a vera causa. Mr. Harker has clearly shown that, 

 as applied to a mass like the Carrock-Fell gabbro, it breaks down 

 hopelessly when subjected to a quantitative test. The principle of 

 Gouy and Chaperon is even more unsatisfactory. Durocher's liipia- 

 tion theory is, perhaps, more promising, but until it has been proved 

 by actual experiment that there is a real analogy between baths of 

 fused metals and silicate-magmas, it can not be said to rest upon an 

 assured basis. Faraday's researches on lead glass "certainly suggest 

 that gravity may act differentially on the constituents of silicate- 

 magmas, independently of the principle of Gouy and Chaperon. Thus 

 he found that glass taken from the top of pots not more than 6 inches 



