THE KINDS OP ROCKS 53 



proceed, and further remarks on the subject may well be de- 

 fei'red until we come to a discussion of individual groups. In- 

 deed, in the present, transitional state of knowledge regarding 

 the chemical and mineralogical composition of rocks, their struc- 

 tural features, and methods of origin, no scheme of classification 

 can be advanced that will prove satisfactory in all its details. 

 The older systems, which were made to answer before the intro- 

 duction of the microscope into geological science, are now known 

 to be founded upon what were in part false, and what have 

 proven to be wholly inadequate, data. This is especially true in 

 regard to eruptive rocks. The time that has elapsed since this 

 introduction has been too short for the evolution of a perfectly 

 satisfactory system ; many have been proposed, but all have been 

 found lacking in some essential particulars. To enter upon a 

 discussion of the merits and demerits of the various schemes 

 would obviously be out of place here, and the student is re- 

 ferred to the published writing! of Naumann, Senft, Von Cotta, 

 Eichtofen, Vogelsang, Zirkel, Rosenbusch, Michel-Levy, Cred- 

 ner, Jukes Brown, and Geikie, as well as those of the American 

 Geologists, Dana,^ Wadsworth,^ and Iddings.^ In the scheme 

 here presented the writer has aimed to simplify matters so far 

 as is consistent with observed facts, and has not hesitated to 

 adopt or reject any such portions of proposed systems as have 

 seemed desirable. 



All the rocks forming any essential part of the earth's crust 

 are here grouped under four main heads, the distinctions being 

 based upon their origin and structure. Each of the main di- 

 visions is again divided into groups or families, the distinctions 

 being based mainly upon mineral and chemical composition, 

 structure, and mode of occurrence. We thus have : 



I. Igneous Eocks : Eruptive, — Rocks which have been brought 

 up from below in a molten condition, and which owe their pres- 

 ent structural peculiarities to variations in conditions of solidi- 

 fication and composition. Having as a rule two or more essential 



^ On Some Points in Lithology, Am. Jour, of Science, Yol. XVI, 1878, pp. 

 335 and 431. 



^ On the Classification of Bocks, Bull. Mus. Comp. Zool. Harvard College, 

 No. 13, Vol. V; also Lithologieal Studies. 



^ The Origin of Igneous Eocks, Bull. PMlosopMcal Society of Washington, 

 1892. See also Quantitative Classification of Igneous Eocks, by W. Cross, 

 J. P. Iddings, L. V. Pirsson and H. S. Washington, Chicago, 1903. 



