CAMBRIDGE ESSAYS : HOPKINS^ ''gEOLOGy/*' 



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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 iuvesti- 

 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 1 ^Y. for 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. 



