ATMOGENIC ROCKS 279 



placed at the ultra basic end of the scale, being entirely free from 

 SiOo when pure. (See Kemp-17.) 



Mctauwrphic Derivatives of Igneous Rocks. 



These comprise: (1) Gneisses, which include granite gneiss, 

 syenitic gneiss, dioritic gneiss, gabbroitic gneiss, pyroxenitic gneiss, 

 peridotitic gneiss, according to the nature of the pyrogenic rock 

 from which they have been derived. (2) Schists, including some 

 mica schists due to crushing and shearing of acid igneous rocks, 

 hornblende schists derived from basic igneous rocks ; chlorite and 

 perhaps talc schists. (3) Serpentines and some soapstones derived 

 from basic igneous rocks. The weathering products of igneous 

 rocks will be discussed at some length in a later chapter. 



2. The Atmogenic OR Atmospheric Rocks (Atmoliths). This 

 group is represented by snow and snow ice,* which are precipitates 

 from the atmosphere, as rock salt and gypsum are precipitates from 

 the water. It forms a surface rock of considerable extent at the 

 present time in the Polar regions, while at various periods in the 

 earth's history it has extended widely over regions now for the 

 most part freed from it. The importance of snow ice in stratigraphy 

 is emphasized by the preservation in it of animals in an almost un- 

 altered condition and by the glacial deposits to which it has given 

 rise in various geologic periods. In composition ice is simple, con- 

 sisting throughout of a single mineral mass, and in texture it is 

 granular crystalline, though appearing compact with conchoidal frac- 

 ture. The types here comprised are : I., Snow ; II., Firn ; and. III., 

 Glacier Ice. 



I. Snozv. This is a loose aggregate of needles and flakes of crys- 

 talline form and structure, precipitated in this state from the atmos- 

 phere in which the material was held as water vapor. The crystal- 

 line form is soon lost through partial melting and evaporation, a fine 

 granular powder resulting. This is the first step in metamorphism 

 (diagenism). 



II. Firn, or Neve. This is an aggregate of snow grains, either 

 loosely held together or united by ice cement. These aggregates are 

 full of air bubbles. Firn is formed below the snow line and repre- 

 sents a further step in metamorphism of the snow. 



III. Snow Ice, or Glacier Ice. This is .a granular crystalline 

 mass in which the individual crystalline ice grains range from the 



* Water ice which is to be considered an igneous rock resulting from the 

 cooHng of a magma is of httlc stratigraphic importance. (See above, p. 278.) 



