48 PRINCIPLES OF PALAZONTOLOGY. 
In the first place, it is perfectly clear that if we admit the 
conception above mentioned of a continuity of life from the 
Laurentian period to the present day, we could never rove 
our view to be correct, unless we could produce in evidence 
fossil examples of a// the kinds of animals and plants that 
have lived and died during that period. In order to do this, 
we should require, to begin with, to have access to an abso- 
lutely unbroken and perfect succession of all the deposits 
which have ever been laid down since the beginning. TI, 
however, we ask the physical geologist if he is in possession 
of any such uninterrupted series, he will at once answer in the 
negative. So far from the geological series being a perfect one, 
it is interrupted by numerous gaps of unknown length, many 
of which we can never expect to fill up. Nor are the proofs 
of this far to seek. Apart from the facts that we have hitherto 
examined only a limited portion of the dry land, that nearly 
two-thirds of the entire area of the globe is inaccessible to 
geological investigation in consequence of its being covered 
by the sea, that many deposits can be shown to have been 
more or less completely destroyed subsequent to their depo- 
sition, and that there may be many areas in which living beings 
exist where no rock is in process of formation, we have the broad 
fact that rock-deposition only goes on to any extent in water, 
and that the earth must have always consisted partly of dry 
land and partly of water—at any rate, so far as any period of 
which we have geological knowledge is concerned. There 
must, therefore, always have existed, at some part or another 
of the earth’s surface, areas where no deposition of rock was 
going on, and the proof of this is to be found in the well- 
known phenomenon of “zzconformability.” Whenever, namely, 
deposition of sediment is continuously going on within the 
limits of a single ocean, the beds which are laid down succeed 
one another in uninterrupted and regular sequence. Such 
beds are said to be “‘ conformable,” and there are many rock- 
groups known where one may pass through fifteen or twenty 
thousand feet of strata without a break—indicating that the 
beds had been deposited in an area which remained continu- 
ously covered by the sea. On the other hand, we commonly 
find that there is no such regular succession when we pass 
from one great formation to another, but that, on the contrary, 
the younger formation rests “ unconformably,” as it is called, 
either upon the formation immediately preceding it in point of 
time, or upon some still older one. The essential physical 
feature of this unconformability is that the beds of the younger 
formation rest upon a worn and eroded surface formed by the 
