'646 GEOLOGY, 



(3) Distinctive deposits. 



Organic rocks. — In the chapter on the origin and descent of rocks, 

 a group of rocks formed directly from organic matter is recognized and 

 described. The chief of these are peat, hgnite, bituminous coal, anthra- 

 cite, and graphite. It is the behef of many geologists that natural gases, 

 oils, and asphalts are also mainly derived from animal and vegetal 

 remains. An alternative view, advocated by Mendelejeff and Moissan, 

 assigns the oils, gases, etc., in part at least, to deep-seated carbides to 

 which water has gained access and developed hydrocarbons, after the 

 analogy of acetylene.* Whatever may be the truth relative to inorganic 

 action, it is clear from geological conditions that some of the natural 

 gases and oils are organic products. Besides the more common organic 

 deposits, there is a long list of minor products, among which are 

 amber, copalite, paraffine, ozocerite, camphene, etc. Guano and copro- 

 lites represent the excrementitious class. 



Inorganic rocks due to life. — Besides these deposits of organic matter, 

 or of its decomposition products, there is a large class formed from the 

 inorganic matter that served auxiliary functions in the economy of life, 

 such as shells, skeletons, etc. For the greater part these are composed 

 of calcium carbonate, and give rise to limestones, marls, chalk, etc. Not 

 a few, however, are silicious, and give rise to flints, cherts, and silicious 

 earths. Some are formed of calcium phosphate, and a few of other 

 inorganic material. The deposits formed in these ways have been 

 defined in the chapter on rocks. 



Fossils. 



The term fossil is used so comprehensively as to include not only 

 the remains of plants and animals themselves, but their tracks, impres- 

 sions, casts, replacements, and all other distinct traces. It also embraces 

 nests, borings, implements, and other distinctive products. These enter 

 into the formation of the two classes of rocks just considered, but they 

 have an independent function. They constitute the specific record of 

 life, and their study not only reveals much of the past history of plants 

 and animals, but furnishes one of the most important means by which 

 the ages of formations are determined. In the early development of 

 the science it was found that the uppermost and hence the latest beds 



1 Science, Vol. VI, p. 838, 1897. Zeitschrift fiir Anorganische Chemie, 1897. 



