A. Holmes — Classification of Igneous Rocks. 115 



been due to the following causes. The end of Palaeozoic time coincides 

 with a worldwide period of mountain-building — the period of the 

 so-called Appalachian revolution of American authors, and that of 

 the building of the Ural and Donetz ranges of Russia (where the 

 change of facies is surprisingly abrupt — the limestone strata of 

 the Upper Carboniferous giving way to the Permo-Carboniferous 

 characterized by great expanses of sandstones). When the limestones, 

 laid down in shallow and comparatively calm waters, were deposited, 

 the environment was favourable to the life of Kugosa Corals. But 

 the mountain-building uplifts, causing great quantities of clastic 

 material removed by denudation to be brought down and deposited 

 in the sea, created conditions the reverse of favourable to the corals. 

 In the Permo-Carboniferous the Rugosa are already rare, and after 

 that time are extinct ; and nearly the same fate befel the Tabulate 

 corals. When suitable conditions again arrived, a new race of corals 

 arose — the Hexacorals — replacing the Rugosa. 



The above explanation of the organization of the Rugose Corals 

 results from considering the simple primary factors of morphogenesis. 

 I think that it is only for such simply organized animals as the 

 Ccelenterata that such a complete and harmonious explanation is 

 possible; and that it is not possible to give an all-embracing 

 expression, reduced to a mathematical formula, of the structure of 

 higher animals, because of their complex organization. In a few 

 cases, however, such expressions have been found for isolated organs 

 of higher animals; for instance, in the case of the development of the 

 feet and teeth of Vertebrates. 



EXPLANATION OF PLATE VIII. 



The Lateral Attachment of Eugose Cokals. 



Fig. 1. — Cyafhophylhim ceratites, Goldfuss, attached to a branch of C. ccespi- 

 tosum, Goldfuss. Devonian : Timan Eange, Eiver Uehta. 



,, 2. — Pseudocaninia conica (Fischer de Waldheim), attached to a spine of 

 ArchcBocidaris. Upper Carboniferous : Mjatshkovo village, Govern- 

 ment of Moscow. 



,, 3. — Petraia permiana, Nechaev, attached to the dorsal valve of Stropha- 

 losia. Permian : Gorodistshe village, Government of Vjatka. 

 A. V. Nechaev's specimen. 



,, 4. — Cyathophyllum heterophyllum, Edwards & Haime, attached by a 

 highly developed, sole -like appendage. Devonian: Paffrath, 

 Ehenish Prussia. 



IV. A MlNERALOGICAL CLASSIFICATION OF IGNEOUS ROCKS. 



By Arthur Holmes, A.E.C.S., D.I.C., B.Sc, F.G.S. 



DURING the past two years the teaching collection of rocks in the 

 Geological Department of the Imperial College has been under 

 re-arrangement, and in the course of the work the writer has had 

 occas ; on to consider very carefully the principles on which igneous 

 rocks should be classified. He is aware that in the present state of 

 our knowledge any such classification must be tentative and experi- 

 mental, and should be judged according to its general convenience, 

 both for teaching purposes and for understanding the various problems 

 that arise from penological studies. The following article summarizes 



