Chap. XX.] 
CONCLUSION. 
505 
considerable distances by land, or for hundreds of miles over the sea in 
floating icebergs. Now, of the far removal of such gigantic blocks we 
have no clear evidence in any preceding geological period ! On the con- 
trary, whilst most large boulders of the Primary, Secondary, or older 
Tertiary rocks bear on their surfaces the signs of having been water- worn 
or rounded by aqueous or atmospheric agency, and have been transported 
to comparatively short distances from their parent rocks (like the con- 
glomerates of the Old Red Sandstone of Scotland), the huge erratics of the 
later cold period (gigantic in comparison with all that preceded them) are 
often angular, or nearly in that state in which they left the mountain- 
side — before, in short, they were wafted over seas or lakes, to be dropped, 
hundreds of miles from their parent rocks, upon submarine sediments, 
which by subsequent elevation have been made portions of our continents. 
Hence, independently of the indications of a more equably diffused and 
warmer temperature in older times than at the present day, such large er- 
ratics are in themselves decisive testimonials of that intense cold which, it 
is believed, was principally due to the increase of great elevated masses of 
land specially characteristic of the quasi-modern period*.- 
Eeceding backwards from this glacial phenomenon f , which, continuing 
into our own times, has been so skilfully illustrated by eloquent writers, 
it is specially my province to impress upon the reader the importance 
of endeavouring to form an estimate of the physical geography of the 
earth during those remote periods when the Palaeozoic deposits were ac- 
cumulated. If, as I firmly believe, lofty mountains did not then exist, we 
have, indeed, in this single condition what may have been one of the 
chief causes of that equable, if not warmer, climate the presence of which 
seems an indispensable hypothesis to harmonize the facts recorded in this 
volume ; and if we add the inference adopted by many philosophers and 
geologists, that the earth, in cooling down from its original molten state, 
must, during immensely long succeeding ages, have gradually lost its heat 
over the whole surface, we are enabled, by reference to physical changes 
alone, to satisfy ourselves that we have in them the chief elements required 
to explain many climatal results. 
Finally, although this rapid survey of the changes of the earth is but 
the outline of a picture which must be filled up by an assiduous study of 
the works of nature during ages yet to come, let me say that it has not been 
attempted without deliberate consideration and extensive researches, du- 
* As I do not believe that the blocks in the pinged before they were finally deposited on the 
Old Red Conglomerates of Scotland were trans- old sea-bottoms. Some authors of distinction, 
ported by ice, so I am not called upon to enter however, view these physical operations as indi- 
into the ingenious cosmical and astronomical cative of two distinct glacial periods, separated 
theory of Mr. Croll, which would account for the from each other by a much warmer period, in 
existence of Palaeozoic glaciers. See Appendix S. which forest trees and large quadrupeds flou- 
t In mentioning the Glacial Period, I embrace rished ; and it has even been inferred that Man 
in one long epoch the formation of vast glaciers existed before the last of these icy epochs. The 
over large parts of the then snow-covered regions reader will find this latter view (to which I do not 
of the earth, as well as the melting of the same, yet assent) eloquently and attractively described 
when their debris, including huge icebergs and by Professor Charles Martins, of Montpellier, in 
erratic blocks, were transported to enormous dis- the ' Revue des deux Mondes,' 1867. 
tances, often striating the rocks on which they im- 
2l 
