ENDOGENETIC ROCKS 271 



within this class is as illogical and unsatisfactory as it well could be. 

 Ill, Crystalline slates { Krystalliue Schicfcr). This includes the 

 j^neisses. mica schists, talc schists, chlorite schists, amphibole and 

 pyroxene rocks, serpentines, the metamorphic calcareous, mag- 

 nesian, and iron rocks, and emery. These, then, include metamorphic 

 sedimentaries as well as metamorphic igneous rocks, and this group- 

 ing can be excused only on th'e ground of convenience. Rosenbusch, 

 indeed, tries to separate these metamorphics of diverse origin by 

 proposing the prefix ortho for those derived from igneous, and para 

 for those derived from sedimentary rocks. To the above three 

 groups is added a fourth, that of the original crust of the earth, 

 Erste Erstarrungs Kriiste. We owe to Johannes Walther (30) the 

 first comprehensive attempt at a truly genetic classification of all 

 rocks. One of the essential features of this system is the recognition 

 of the fact that metamorphic rocks belong with the rocks from 

 which they are derived. He proposed four main divisions, namely: 

 I, mechanical or clastic ; II, chemical precipitates and sublimates ; 

 Til, organic; and, IV, volcanic rocks, or consolidated lavas. 



This is essentially the classification adopted in this book, except 

 that a stronger distinction is made between the first group, the 

 E.vogenctic, or clastic, and the others which are classed as Endo- 

 genetic (Grabau-9). The Endogenetic are those formed by agents 

 acting from within, i. e., by agents intimately associated with the 

 rock mass forming ; they produce rocks by solidification, precipita- 

 tion or extraction of the mineral matter from the states of igneous 

 fusion, aqueous solution or vaporization. The Exogenetic rocks, on 

 the other hand, are those formed by agents acting from without 

 upon already existing rock matter, reducing it to a finer condition 

 either by mechanical or by chemical means, while still leaving it in 

 a solid state. They are, in fact, the clastic rocks of earlier writers, 

 while the Endogenetic rocks are the non-clastic. 



I. The Endogenetic Rocks. 



These are best understood when we approach them through a 

 consideration of the material of the earth as a whole, and the con- 

 ditions under which it appears to us. Aside from the solid state, 

 three general conditions may be conceived of under which the ma- 

 terial of the earth's crust may exist. These are: i, the molten con- 

 dition, or state of igneous fusion and solution ; 2, the state of solu- 

 tion in water; and. 3, the state of vapor or incorporation in the air. 

 A molten magma is not merely produced by a change of condition 



