108 DYNAMICAL GEOLOGY. 



things into consideration, it has been estimated that one 

 quarter to one half inch per annum is a large estimate of 

 rate of ground-rising. The subsidence can not be greater 

 and may be much less than this. At this rate a subsi- 

 dence of 10,000 feet would require 250,000 to 500,000 

 years. The whole of this, however, must not be accred- 

 ited to the present geological epoch. It probably extends 

 back into the Tertiary. 



Geological Application. 



There are several points in the preceding discussion 

 which throw important light upon the structure and his- 

 tory of the earth. 1. We have here examples of lime- 

 stone rock, formed by coral agency over millions of square 

 miles, and in places many thousand feet thick. For not 

 only is limestone formed on the site of the reefs (reef -rock), 

 but the fine coral debris is carried by waves and currents 

 and strewed over the whole intervening space. We find 

 thus a key to the extensive deposits of limestone formed 

 in previous geological times. 2. The kind of rock formed 

 also deserves attention. The reef-rock is, in some parts, 

 a coral breccia; in other parts it consists of rounded 

 granules, cemented together (oolite). In the deep sea of 

 the intervening spaces, the bottom ooze is a fine coral mud, 

 which, dried, looks much like chalk, and by some has 

 been supposed to be indeed the modern representative of 

 chalk ; but more probably, it hardens into a compact 

 limestone. Now, in limestones of previous geological 

 epochs, we find similar structures ; i. e., extensive fine 

 limestone, with areas of coarse coral breccia or of oolites. 

 We are thus able to determine the position of old coral 

 seas and the lines of old coral reefs, even though they are 

 now occupied by mountain-ranges, as in the case of the 

 Jura Mountains (Heer). 3. Lastly, we have here ex- 

 amples of movements of the earth's crust on a grand 



