48 PRINCIPLES OF PALAEONTOLOGY. 



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 prove 

 our view to be correct, unless we could produce in evidence 

 fossil examples of all 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. If, 

 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 "unconformabUity? Whenever, namelv, 

 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 " un con form ably," 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 



