BIOLOGICAL CHANGE IN GEOLOGICAL TIME. 



95 



for a similar conclusion. The extent of Jurassic rocks removed 

 from the region of these islands, not including sea areas, may 

 be estimated at about 100,000 square miles, of Cretaceous rocks 

 110,000 square miles, and of Tertiary deposits 115,000 square 

 miles, the total area of the British Islands being 121,700 square 

 miles. So that in this region alone only a very small proportion 

 is left of these once wide-spreading Neozoic groups of formations. 



Again, in Ireland the Coal Measures evidently once covered 

 the whole of its interior area, while now it is found that almost 

 the whole of those deposits, extending over fully 16,000 square 

 miles, and containing most valuable beds of coal, have been re- 

 moved and swept into the sea. The thickness of these destroyed 

 rocks was very great also. Professor Eamsay estimated that fully 

 10,000 feet thickness of Lower Silurian (not Coal Measures) slate 

 had been removed from what is now the summit of Snowdon. 



The great unconforniabilities and lacunae are other obvious 

 illustrations of the imperfection of the geological record. In 

 Somersetshire, the Carboniferous Limestone, highly inclined, is 

 succeeded immediately by horizontal Inferior Oolite. Under 

 the London area Cretaceous rocks lie upon Devonian, while 

 below Dover the Coal Measures have been reached immediately 

 below Jurassic rocks. 



Of the animals and plants living at the time of the deposition 

 of the various sedimentary rocks of the globe, only a small 

 proportion have left fossil remains, even of marine testacea, and 

 of land animals and plants very few indeed, for the great bulk 

 of marine shells would be broken up and destroyed by wave 

 action, while of terrestrial animals and plants only the remains 

 of those would be preserved that escaped decay and decompo- 

 sition by entombment under exceptionally favourable conditions 

 for their preservation. And finally, it must be remembered 

 that only in a few places, each of very limited area, and aggre- 

 gating altogether not one-millionth of their extension at the 

 surface, have the sedimentary rocks been carefully examined. 



But very imperfect as this record of the rocks undoubtedly 

 is, it gives, as far as it goes, a true revelation of the successive 

 faunas that have peopled, and of the successive floras that have 

 clothed th.e globe. What it tells us is therefore so much positive 

 knowledge of the highest value, although it be but a fragment 

 of the great story of Creation. 



GEOLOGICAL Time. 



An adequate consideration of the causes of biological changes 

 also requires attention to the duration of geological time, for 



