26 EVOLUTION 
rocks came into existence gradually and through the 
operation of a supposed natural cause. 
A great advance upon Werner’s theory was made 
by Hutton, who, observing the formation of strata 
at the present day from the sediment washed down 
by rivers, concluded that the ancient strata were 
deposited in the same manner. Since, by the long 
continuation of this process the continents must 
gradually become reduced to the level of the sea, 
Hutton supposed that at long intervals of time the 
action of subterranean heat came into play, and 
fresh continents were upheaved, a process accom- 
panied by the outpouring of the igneous rocks, 
the true origin of which he had duly recognised. 
In this theory a hypothetical cause still survives, since 
we have no actual experience of vast upheavals of the 
kind which Hutton supposed to have taken place. 
Lyell showed that such slight changes of level as are 
known to be in progress at the present day, especially 
in association with the phenomena of earthquakes, 
might, if continued over a long series of ages, give 
rise to the necessary amounts of elevation. Lyell also 
pointed out a number of subsidiary causes of dis- 
integration and deposition of strata of the. kind which 
can still be seen in operation at different parts of the 
earth’s surface. At the present time it is sometimes 
thought that Lyell went a little too far in his cham- 
pionship of the cause of uniformity. Lyell supposed 
that the agencies which may now be everywhere ob- 
served, in operation, such as rain and rivers, the sea, 
volcanoes and earthquakes, were sufficient to account 
