272 PRINCIPLES OF STRATIGRAPHY 



from the solid to the liquid state through a rise in temperature 

 (i. e., snow or ice to water), though this is undoubtedly to be re- 

 garded as the simplest state of fusion. A much more complex state, 

 however, exists in the fusion of compound materials such as the 

 rock magmas consist of. These are to be regarded as solutions of 

 the silicates one in another or several in one, under a condition of 

 high temperature. Solutions of salts in water are not actually dif- 

 ferent in kind from these "igneous solutions," though they take 

 place at a lower temperature. Since, however, such a large part of 

 the earth's surface consists of water in the liquid state, it is con- 

 venient to consider solution in water as distinct from igneous solu- 

 tions. This conception makes it possible to identify the three states 

 of the material from which the lithosphere is to be formed, re- 

 spectively, with the pyrosphere, hydrosphere, and atmosphere. By 

 a direct crystallizing out from the three states (or precipitation in 

 the amorphous state), we have three groups of endogenetic rocks 

 composing the lithosphere, namely: i, the pyrogcnic, or igneous; 2, 

 the atmogenic, or atmospheric ; and, 3, the liydrogenic, or aqueous 

 (in a restricted sense). By the physiological activities of the organ- 

 isms which constitute the biosphere, the biogenic or truly organic 

 rocks are formed and these may be divided into ^oogenic, or animal- 

 formed, and phytogenic, or plant-formed. These four types of 

 endogenetic rocks may then be tabulated as follows : 



1. Pyrogenic, or igneous, rocks — Pyroliths. 



2. Atmogenic, or atmospheric, rocks — Atmoliths. 



3. Hydrogenic, or aqueous, rocks (in a restricted sense) — 

 Hydroliths. 



4. Biogenic, or organic, rocks (in a restricted sense) — Bioliths. 

 The origin and interrelations of these types are shown in the 



following diagram (Fig. 38). 



The first of these groups comprises the Igneous rocks of most 

 current classifications, the other three are commonly included with 

 the clastic rocks as sedimentaries. Metamorphic rocks, though gen- 

 erally considered independently for convenience sake, belong strictly 

 with the unaltered rocks from which they are derived, and of which 

 they constitute modifications. The chief practical difficulty in plac- 

 ing them with their unaltered prototypes in a scheme is found in 

 the impossibility of determining the precise types from which they 

 have been derived (see further below). 



The further subdivisions of the Endogenetic rocks are pri- 

 marily based on chemical composition, and secondarily on texture. 

 As a rule, composition is most readily expressed in terms of the 

 minerals composing the rock if they can be ascertained, and this is 



