1879.] of life forms to the breaks of continuity in the strata. 247 



gained, so that we cannot tell how far one district supplements 

 another. 



If a continental area with its various rocks were to go down 

 and the sea to cover with sediment the irregular surface, filling 

 up hollows and creeping up hills, and if we could after all this has 

 happened get a clean cut through this new continuously-deposited 

 sediment to the old rocks on which it was laid, though the 

 material might be of the same kind as that which made up the 

 older rocks, we should find evidence that the older series had been 

 upheaved, had suffered denudation and gone down again before the 

 newer series had begun to accumulate over it, and we should see 

 that a long time must have elapsed between the formation of 

 those old rocks and the earliest part of the newer deposit, that 

 there had been an interruption in the geographical conditions, 

 and we should say there was a br^eak between the two series. 



On the other hand, if Ave examined the newer sediment itself we 

 should find that although it was made up of various material, here 

 a pebble-beach, there a mudbank, in one part a coral reef, in 

 another a heap of shells, still that it all was formed continuously 

 during a period of depression, i.e. that there was no break in the 

 series. That is to say, difference of lithological character does not 

 involve lapse of time as does an unconformity, and so when we are 

 considering the changes in the forms of life, between two dissimilar 

 rocks, Cretaceous and Eocene for example, we must remember that 

 there is not in this difference any evidence of a break in time, 

 such as we find between Silurian and Upper Old Red, or between 

 Carboniferous and Mercian, but the conditions which gave rise to 

 the formation of chalk with its myriads of microscopic marine 

 organisms were very different from those which allowed the accu- 

 mulation of the estuarian and fluviatile beds of the Lower Eocene, 

 such changes in sediment being the usual sudden effects of gradual 

 operations such as the silting up of hollows, destruction of head- 

 lands, and such' like. 



A period or area of upheaval is essentially one of destruction, 

 and the removed material is carried to the areas of depression 

 for that period. So where we find in the rocks evidence of .vast 

 masses gone, we cannot there find traces of the life of the period, 

 as there is there no sediment in which its remains could be pre- 

 served. 



It is convenient to have a table of the known strata, and 

 although we cannot arrange all the rocks of thS world in parallel 

 columns, and say that ABC of -one area are exactly synchronous 

 with A'B'C of another, still if we take any one country and establish 

 a grouping for it, we find so many horizons at which equivalent forma- 

 tions can be identified in distant places that we can generally make 

 an approximation to homotaxis as Huxley called it. The most- 



Vol. III. Ft. yi. 18 



