28o PRINCIPLES OF STRATIGRAPHY 



size of a pea near the firn border to that of a hen's egg. The crys- 

 tals are intimately united, so that in fresh ice they cannot be dis- 

 tinguished. 



3. The Hydrogenic Rocks (HydroHths). These are the only 

 true aqueous rocks and they are wholly of chemical origin. Though 

 often forming beds of considerable extent, tliey do not generally 

 enclose evidence of their age. though in some cases organic re- 

 mains are found. As a rule, however, the age of a bed of rock salt 

 or gypsum can be determined only by the age of the enclosing 

 rocks. Chemically formed, or hydrogenic limestones, on the other 

 hand, may include organic remains, as is the case in calcareous tufa 

 and cave deposits. Many great limestone beds which are sparingly 

 fossiliferous have frequently been considered of chemical (hydro- 

 genic) origin, but it is not improbable that many, if not most, of 

 these will prove to be of clastic origin. Among hydrogenic 

 rocks of less stratigraphic importance are siliceous sinters and 

 their alteration products and the various ferrites or iron ore 

 deposits, which owe their precipitation from water to inorganic 

 reactions. 



4. The Biogenic Rocks (Bioliths). These are the only true 

 organic rocks, /. c, those due directly to the physiological activities 

 of organisms. They are among the most important rocks with 

 which the stratigrapher has to deal, for they are the rocks richest 

 in organic remains. As examples may be mentioned the organic 

 limestones, such as coral and shell rock ; and the organic oozes, 

 whether calcareous or siliceous. Here also belong the deposits of 

 coals and vegetable materials in all stages of carbonization, from 

 peat to anthracite. 



Bioliths may be divided into Zodliths, or those of animal origin, 

 and Phytoliths, or those of vegetal origin. The latter include two 

 important types, those formed by the direct accumulation of vegetal 

 matter, such as peat, coals, etc.. which, from their burnable nature, 

 have been called caustobiolitJis^ (Potonie. see Chapters x and xi), 

 or better caiistophytoliths ; and those forming pure accumulations 

 of mineral matter, either silicia (diatoms) or lime (nullipores, etc.). 

 These are the a causto phytoliths. Caustobioliths may also be of 

 zoogenic origin (caustozooliths), but these are rare (some oils, etc.). 

 Finally, caustoliths of non-organic origin are also known, such as 

 chemically formed petroleum, asphalt, graphite, and sulphur. Caus- 

 tobioliths have been divided into three groups, according to their 

 mode of origin. These are : 



* Koi'(rT6s, verbal adjective of YiaUiv — burn. 



