8o GEOLOGICAL BIOLOGY. 



were preserved, and this condition is found only under 

 muds, in marine conditions; in the bottom of lakes or in 

 river bottoms fossilization may take place, but the fossils 

 are then liable to some change of composition. Fossils pre- 

 served under the most favorable conditions, by long-contin- 

 ued pressure and the slight circulation of fluids in rocks, 

 suffer change after their formation, particularly in the way of 

 assuming a crystalline structure. 



The Majority of Fossils are of Marine Organisms. — From the 

 above remarks it is evident that the larger proportion of fossils 

 must be those preserved under the surface of the ocean ; 

 next will be found those buried in land basins covered by 

 fresh water; and only very rare will be the cases of fossils 

 otherwise preserved. Hence marine organisms will naturally 

 present in the rocks the fuller records of their history : fresh- 

 water or brackish-water species will be recorded less perfectly ; 

 and the organisms normally living under land or air condi- 

 tions will be recorded in fossils very imperfectly at the best. 

 The great majority of even the hard parts of such organisms 

 must be destroyed before reaching the position of a safe burial- 

 place, and our studies will be directed by this law of preser- 

 vation. Marine organisms, and largely marine invertebrates, 

 will be selected as illustrations of the laws of the history of 

 organisms, because the records regarding these are fuller than 

 regarding any other kinds of organisms. 



Various Kinds of Fossils enumerated. — To the question 

 ** What are fossils?" the concise answer is: Fossils are 

 traces of organisms buried in the rocks. A full definition 

 would be a descriptive treatise on Paleontology. As to their 

 forms, fossils are as various as are organisms. A useful analy- 

 sis, however, may be made of their composition. Fossils 

 are composed (A) either of the original materials of the organ- 

 ism which made and left them ; they are then strictly remains 

 of dead organisms, or of parts of them. Or (B) fossils may be 

 casts or moulds in the rocks where these structures were origi- 

 nally buried and afterwards removed. (C) The filling of the 

 cavities thus formed constitutes other kinds of fossils, (i) 

 The cavity may be filled by mineral matter carried in by infil- 

 tration and redeposited ; (2) the cavity may be filled through 



