CAMBRIDGE ESSAYS : HOPKINS* "GEOLOGY." 515 
to any cause which can properly be included in those secondary causes 
which, in their operations and effects, constitute what we term nature, 
in the usual acception of the term ; or are they to be referred to 
some higher order of causation, which may be best represented as a 
more immediate act of creative power ? 
In the discussion of these questions it is essential to enter upon other 
collateral physical questions themselves important points of investi- 
gation. " The great all-pervading physical agent to which the changes 
of this earth's condition are due, is heat.^' It is heat which vaporises 
the waters of the ocean, and lifts them above the mountain-tops, thence 
again to descend laden with sediment to their native bed ; and to the 
same cause we must trace the winds which raise the waves, and give to 
them their power to undermine and cast down the loftiest cliffs. A 
large part of the denudation, therefore, is due to this cause, and it is to 
heat alone, under any view of the subject, that we can possibly assign 
the agency by which continents and mountains have been elevated, 
while its milder influences are traceable in the past changes of climate 
in certain regions at least, of which geology bears no doubtful evidence. 
Within a depth of from fifty to eighty feet from the surface, the 
terrestrial mass is affected by the change of temperature from one 
season of the year to another, the effect becoming less sensible as we 
recede, until it becomes so small that a thermometer, placed at the 
depth above mentioned, indicates the same temperature at all seasons of 
the year. The temperature at this limiting depth exceeds by about 
one degree the mean annual temperature of the ground just below the 
surface, and that of the atmosphere just above it, and within this 
limiting depth the variation of temperature is due to entirely solar 
influence. But the point with which we are immediately interested is 
the law of temperature below this limiting depth of fifty to eighty feet. 
The rate at which the temperature increases as we descend, varies in 
different localities ; but where the depths are great we find a close 
approximation to a common rate of increase which, as determined by 
the best observations in the deepest mines, shafts, artesian wells in 
western Europe, is very nearly IT. /or an increase in depth of sixty feet. 
In the consideration of the changes of climate which have taken 
place at different geological periods, the author dwells on those charac- 
ters of fossils, shells, plants, &c., by which the extinct organisms are 
related to particular existing geographical groups, or to particular local 
faunas and floras, geological evidence being considered as generally in 
favour of former higher temperatures. There is no distinct evidence, 
however, to show whether change of climate was or was not accom- 
panied by any oscillation of temperature during the Palaeozoic and 
Secondary Periods ; but when we come to a portion of the later 
Tertiary Era — the glacial period — we find evidence proving a large 
portion of Western and Northern Europe to have been, during the 
period in question, considerably colder than at present, and consequently 
of considerably lower temperature also than that of the preceding 
periods, thus establishing a large oscillation of temperature in reference 
to this particular region. The fossil fauna, too, of the glacial period, 
bears a decidedly Arctic aspect. 
