REVIEWS—GEOLOGICAL EVIDENCES O¥ MAN’S ANTIQUITY. 379 
- wards the solid state, and eventually passing into this, as regards the 
surface of the earth-mass. Although the rock-matters resulting 
from the first consolidation, must long have disappeared, or have 
lost altogether their original characters, a period would finally arrive 
when a certain degree of stability—or rather a more equal balance 
between destructive and formative forees—would be approached. This 
would arise, when by the continued radiation of heat into space, the 
earth’s crust became sufficiently thick to admit of the condensation of 
water upon its surface. Then a new set of phenomena would appear. 
The exposed rock-surfaces would be slowly worn down by aqueous and 
atmospheric agencies, and the materials, thus obtained, would form 
over the sea-bed a gradually increasing thickness of stratified deposits. 
Many of these rocks, though mostly in an altered or metamorphic con- 
dition, have been preserved to us. They contain no vestiges of organic 
forms, vegetable or animal. Life, as yet, held no place upon the earth; 
and as these strata, even as now seen, present a thickness of many 
thousands of feet, it is evident that this first or Azoic period of the 
Earth’s history was one of almost immeasurable length. 
The busy agents of Decay and Renovation, those old but yet unre- 
conciled antagonists that have made Nature their battle-field from all 
time, still continued their active and unceasing strife. The older rock- 
masses furnished the sediments for the formation of newer strata; but 
in these latter, we find the records of a wonderful change, witnessed 
by the Earth at the close of its azoic day. To the strange mystery 
of the Earth’s presence, the still stranger mystery of Life had now 
been added. The organic remains enclosed within these earliest 
fossiliferous rocks, are of comparatively low types. Fucoids, brachio- 
pods, trilobites, constitute the more characteristic forms: the verte- 
brated life-structure is entirely absent. A little higher in the series, 
a little later in the course of time, plants of terrestial growth, fishes, 
and obscure reptilian types, make their appearance, together with 
powerful tetrabranchiate cephalopods and other forms of an extinct 
or rare organization, as compared with the life-forms of existing 
seas. Strata still succeed strata, as newer sediments are spread along 
shore-lines, in bays, and over the sea-bed. Many of the earlier types, 
or those enclosed in the lower rocks—graptolites, trilobites, and others 
—die out, not gradually as it were, as though the organic pattern 
were changed by gradnal modification, but abruptly, at fixed stages in 
the rock series, before the close even of this first life-period, the great 
Paleozoic Age. To this, and some related points, we shall have oc- 
