504 
SILURIA. 
[Chap. XX. 
with the augmentation of dry lands and the growth of trees, as seen in the 
Devonian system, Pishes abounded, to be followed afterwards by Reptiles. 
Subsequent to the accumulation of the Carboniferous rocks, we have proofs 
that the earth's surface was so powerfully corrugated* that, after many 
Mesozoic perturbations, the groups of animals and plants of the Tertiary 
periods were far more restricted to limited regions and varied climates. 
On the other hand, the numerous and positive evidences of a former 
wide distribution of similar animals and plants enable us fairly to brings 
before our mind's eye the physical geography of those long epochs when 
such large portions of our present continents were under the waters, and 
jungles of succulent and fern-like trees occupied extensive low lands, sub- 
jected during long periods to sinkings and uprisings, both gradual and 
sudden. Not less clearly do we perceive, from other physical evidences, 
the manner in which eruptive forces subsequently breaking out with vio- 
lence after the close of the Carboniferous era, as at many other periods, 
have thrown the strata into those grand undulations and contortions, ac- 
companied by stupendous fractures, which have given to the Palaeozoic, 
and even the Secondary deposits their curvatures and limits. 
Thenceforward was continued that long series of additional and repeated 
emissions of volcanic matter from within, of elevations of sea-bottoms 
and corresponding depressions of land, combined with the metamorphism 
of strata (these changes being often accompanied by corresponding new 
creations of animals suited to the existing conditions), during the formation 
of the Secondary and Tertiary deposits. By these great physical operations, 
both gradual and paroxysmal (some of the grandest of which must have 
occurred in recent geological times), our planet was eventually brought to 
possess the climatal relations which have for so long prevailed. That these 
elevations and depressions finally produced a state of things very different 
from that of such former eras, is, indeed, everywhere registered by a mul- 
titude of well-attested data. 
Among the terrestrial changes to which science clearly points, there is 
no one which better deserves to be alluded to in a few parting words, 
than that great mutation of climate, by which extensive fields of ice were 
first, as I believe, formed upon the sea, and large glaciers upon the land. 
As very lofty mountains in moderate latitudes, and masses of land and 
water in Arctic or Antarctic regions, are essentially the seats of glaciers 
and ice-rafts, so we know that these bodies alone have the power of trans- 
porting huge erratic fragments of rocks from their native mountains to 
* I by no means deny that much perturbation the existence of any lofty mountains until the 
prevailed in the earlier stages of the planet, as ex- crust of the earth had undergone some of the sub- 
X^lained in the first two Chapters. On the con- sequent mutations alluded to, particularly those 
trary, I admit the powerful emission of much ig- which followed the accumulation of the Carboni- 
neous matter, the transmutation of sedimentary ferous strata, and which were repeated at so many 
into crystalline rocks, and the emergence of great subsequent periods, notably, and perhaps most 
land-areas, anterior to the accumulation 01 any powerfully, in the Tertiary times which preceded 
of the deposits which are now under conside- the historic era. 
ration ; but I deny that there are indications of 
