Chap. XI.] THROUGnOUT THE WOKLD. 105 



tends to correspond both in their first appearance and 

 final disappearance. 



There is one other remark connected with this subject 

 worth making. I have given my reasons for believing 

 that most of our great formations, rich in fossils, were 

 deposited during periods of subsidence ; and that blank 

 intervals of vast duration, as far as fossils are concerned, 

 occurred during the periods when the bed of the sea 

 was either stationary or rising, and likewise when sedi- 

 ment was not thrown down quickly enough to embed 

 and preserve organic remains. During these long and 

 blank intervals I suppose that the inhabitants of each 

 region underwent a considerable amount of modification 

 and extinction, and that there was much migration 

 from other parts of the world. As we have reason to 

 believe that large areas are affected by the same move- 

 ment, it is probable that strictly contemporaneous 

 formations have often been accumulated over very wide 

 spaces in the same quarter of the world; but we are 

 very far from having any right to conclude that this 

 has invariably been the case, and that large areas have 

 invariably been affected by the same movements. When 

 two formations have been deposited in two regions 

 during nearly, but not exactly, the same period, we 

 should find in both, from the causes explained in the 

 foregoing paragraphs, the same general succession in 

 the forms of life ; but the species would not exactly 

 correspond ; for there will have been a little more time 

 in the one region than in the other for modification, 

 extinction, and immigration. 



I suspect that cases of this nature occur in Europe. 

 Mr. Prestwich, in his admirable Memoirs on the eocene 

 deposits of England and Prance, is able to draw a close 



