the Distribution of Life and of Rocks. 413 



27. Under certain circumstances, when a f2;roup of life is driven 

 to a new locality by elevation, it happens tliat the conditions of 

 least resistance determine its course either over or under the 

 group which previously occupied the ground ; so that a grada- 

 tion of life in zones of depth comes to result fi'om the distribu- 

 tion of life in provinces. 



28. No depression of land can tal<e place without the deposit 

 which was forming furthest out at sea appearing to be newer 

 than the others. Thus here, s, c, I represent the typical sand- 



stone, clay, and limestone. By depression s^ is formed nearer to 

 shore, and c' is deposited over 5 and c; and so the succession 

 is continued, if the depression goes on, till the stratum s^, s^, 

 s^, s is formed under the c series, the c series in its turn being 

 under the / series. Yet this apparent superposition gives a 

 very erroneous idea of the age of the beds; and since the life 

 follows the receding shore, it happens that the fauna of s is also 

 found in s^. 



Hence it follows that neither in the rocks produced by eleva- 

 tion nor depression can the age of the beds be determined by 

 superposition or by fossils. 



29. Whenever a sandstone is superimposed on a clay, in 

 some portion of the area the older stratified rocks will be de- 

 nuded, if they were ever deposited there. Hence if such a 

 sandstone contains extraneous fossils, they came from rocks 

 which existed beyond the sandstone area, and on which the 

 sandstone was not then being accumulated. 



If the sea-shore is stationary, the majority of the fossils, ac- 

 cumulated from the life of the time, will be much worn. 



30. The fauna and flora of the British Isles is not the only 

 known fauna and flora. From the phenomena of elevation and 

 depression, it follows that no fauna or flora can cover more than 

 a fraction of the earth's surface at the same period of time ; 

 though it is quite possible for a fauna to migrate during a long 

 period of time over a far larger area. And this is usually the 

 significance of the correspondence between distribution of life 

 in time and in space ; and by a worldwide fauna is usually un- 

 derstood a fauna that has been split up by physical changes, so 

 that at a few widely divided points a less or greater proportion 

 of fossils (usually few) are found like those of the typical loca- 

 lity, but almost invariably mixed with others unlike those pre- 



