158 
THE GEOLOGIST. 
tell us too of a gradual refrigeration of our planet, and refer many 
problems of former temperature to the ancient higher internal incan- 
descence of our planet. Measure off on a roll the successive massei 
of rock -strata which we know by their superposition to be true in i- 
cations of geological time, on a scale of H inch to a thousand feet of 
vertical thickness, and ^^our diagram will reach to a length of nearly 
nine feet. 
Over the uppermost of these add a segment to represent recent 
deposits ; it will scarcely be the eighth of an inch in thickness. Yet 
four thousand years at least have intervened since the parents of the 
human race trod the verdant floor of beauteous Eden. Take the 
next in order, the latest Tertiary age — the age of glacial drifts and 
icebergs, and a quarter of an inch will overlap the segment you have 
drawn. And yet for thirty thousand years at least the foaming cataract 
of Niagara has been cutting through the raised and consolidated strata 
of that vast age, for vast it must have been when whole species of 
maritime mollusca migi-ated many geographical degrees from their 
ancestral haunts to seek out warmer climes ; when whole continental 
tracts were raised into the regions of perpetual snow ; and again, the 
uplifted lands subsiding to their ancient levels, their shores were once 
more inhabited by the returned posterity of the out-driven shell-fish. 
Take the next segment of the rock- formations, and the caves disgorge 
the bones of hundreds of extinct pachyderms and ruminants. Com- 
pute the cubic space of the fleshly bulk of the collective carcasses of 
those exhumed by inquiring man alone, and their volume far surpasses 
the capacity of the cave to contain them ; and yet for hundreds 
disinterred, thousands remain behind. Take the next age, and two 
thousand feet of sediment tell of still more extensive changes and still 
more extended time ; and the next, and the next ; and greater and 
greater becomes the thickness of the stony volumes of the earth's 
history, until in the coal-measures we have 15,000 feet and more of 
instructing leaves, and in the Silurian and the " bottom-rocks " 
25,000 and 2G,000 feet of evidential records. 
These too are only the records of the periods of active deposit of 
sediment, and the minimum even of that. No indication is here of 
the periods (equally great, or greater) of cessation, nor of the far sur- 
passing periods of re-generation and re-formation. The greatest 
