318 



RESUME AND THEORETICAL DISCUSSION 





There is, however, quite a large development of marine and brackish water 

 Tertiary in the foot-hills at the base of the range toward the southern ex- 

 tremity of the Great Valley. Along the central portion of the Sierra, the 

 Cretaceous occupies more area on the surface than the Tertiary does, but 

 both formations are thin ; and in some places the marine beds are so mixed 

 with detrital and volcanic matter which has been washed down from above 

 bearing at the same time fragments of wood and bones of land animals, that 

 the two formations cannot be separated from each other. 



It being quite certain, then, that the Sierra was above the sea in Tertiary 

 times, and has remained so, also that it had its system of rivers, with their 

 lake-like expansions, it is to be taken for granted that this large area of 

 land, with its bodies of fresh water, would become, in time, the home 

 of various kinds of animals and plants suited to the conditions there 

 presented. But our knowledge of the origin and development of life, 

 and especially of the manner in which groups of species have spread 

 themselves on the earth, is so imperfect that we have no right to be 

 surprised if large areas occur, on which there are no organic remains to 

 be found, as partial testimony of the lapse of geological ages. With r 

 to formations of marine origin we have lees' difficulty, because the ocean 

 has at all times been a continuous body of water, extending over the larger 

 part of the earth's surface ; land, on the other hand, has existed in detached 

 areas, of very small size as compared with the whole body of salt water, 

 and of whose former connections with each other we can, at the present 

 time, have but little idea : hence, when we find a gap in the series of geo- 

 logical formations in any particular region, we are wont to say that during 

 the period represented by the wanting strata the land was raised above the 

 sea. In doing this, we omit to notice that there must be also some reason 

 why subaerial deposits, accompanied by a development of organic life, should 

 not have been formed under such circumstances. It is true that we may 

 take it for granted that erosive action is always modifying the surface of the 

 land, and bringing about, on the whole, a more effective obliteration of the 

 evidence of the lapse of past geological ages than may be expected to take 

 place under the deep water; but even this consideration does not remove 

 all the difficulties in the way of accounting for those great gaps in the for- 

 mations, which present themselves in regions which we may assume to have 

 been above water during certainly a large part of the time, for which there 

 is nothing to show in the way of organic life. As an instance of this, we 



