BREAKS IN THE GEOLOGICAL RECORD. 49 



break in the successions of strata as well. Let us therefore 

 briefly consider how far these interruptions and breaks in the 

 geological and palaeontological record can be accounted for, and 

 still allow us to believe in some theory of continuity as opposed 

 to the doctrine of intermittent and occasional action. 



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 inter- 

 rupted 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 deposition, 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 " unconformability. " When- 

 ever, 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 

 4 



