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 voleanic 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 regard 
to formations of marine origin we have less 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 
aul 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 
