the Distribution of Life and of Rocks. 413 
97. Under certain circumstances, when a group of life is driven 
to a new locality by elevation, it happens that 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 from the distribu- 
tion of life in provinces. 3 
28. No depression of land can take place without the deposit 
which was forming furthest out at sea appearing to be newer 
than the others. Thus here, s, c, / represent the typical sand- 
stone, clay, and limestone. By depression s! is formed nearer to 
shore, and c! is deposited over s and c; and so the succession 
is continued, if the depression goes on, till the stratum s°, s?, 
s!, sis formed under the ¢ 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- 
