Siluria—Present State of Geology. 321 
Paleozoic. But, whether this ancient series be divided into double 
or triple classes (some paleontologists preferring to hold the Devonian 
as a separate and intermediate type), the result of the researches of 
the numerous authors appealed to in this volume has unquestion- 
ably justified the application of the term ‘ system’ to the Silurian 
rocks. 
** At the close of the Permian era, an infinitely greater change 
took place in life than that which marked the ascent from the Silu- 
rian system to the overlying groups. The earlier races then dis- 
appeared (at least all the species), and were replaced by an entirely 
new creation, the generic types of which were continued through 
those long epochs which geologists term secondary or menzoic, (the 
medieval age of extinct beings). In these, again, the reader will 
learn by consulting the works of many writers, how one formation 
- followed another, each characterized by different creatures; many 
of them, however, exhibiting near their downward and upward limits 
certain fossils, which link on one reign of life to another. 
‘ Tn surveying the whole series of formations, the practical geo- 
logist is fully impressed with the conviction, that there has, at all 
periods, subsisted a very intimate connection between the existence, 
or at all events the preservation of animals, and the media in which 
they have been fossilized. The chief seat GF Sutter Tifa te’ eno geo- 
logical epoch is often marked by a calcareous mass, mostly in a 
central part, towards which the animals increase from below, and 
whence they diminish upwards. Thus, the Llandeilo limestone of 
the Lower Silurian, and the Wenlock of the Upper Silurian, are 
respectively centres of animalization of each of those groups. In 
like manner, the Eifel limestone is the truest index of the Devonian ; 
the Mountain limestone of the Carboniferous; and the Zechstein, or 
‘English Magnesian limestone of the Permian. Throughout the 
secondary rocks the same law prevails more or less; and wherever 
the typical limestone of a natural group is absent, there we perceive 
the deposits to be ill characterized by organic remains. For ex- 
ample, the Trias, so rich in fossil contents where its great calcareous © 
centre the Muschelkalk is present, as in Germany and France, is a 
miserable sterile formation in Britain; where, as in our own new 
Red Sandstone, no such limestone exists. 
“s Among the terrestrial changes to which science clearly baht: 
there is no one which better deserves to be recorded in a few part- 
_ ing words, than that great mutations of surface and its accompany- 
ing loss of warmth, by which extensive fields of ice were first 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 now essentially the seats of glaciers 
_ and icebergs; so we know that these bodies alune have the power of 
_ transporting huge erratic blocks from their native mountains to con- 
be siderable distances by land, or for hundreds of miles over the sea in 
| floating icebergs. Now, of the translation of such blocks we have 
VOL. LVII. NO. CXIV.—OCTOBER 1854. % 
