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THE ROCKS AS RELATED TO TIME? 151 
shells, that could only have lived in an arctic climate, and now live only 
in the northern seas. 
2. The Existence of Glaciers in Britain—In the same way, we can as 
conclusively prove that these great bergs were broken from glaciers that 
at that remote epoch filled the upper valleys of the Highlands, to the 
north and south. This is shewn by such facts as these, which we can 
but enumerate—the general aspect of these valleys; the striations and 
polished surfaces pointing always to the highest peaks, whence the ice- 
streams descended ; the rounded rocks with steep sides looking down the 
valleys ; the rocky, worn, and broken débris ; the moraines or rock-walls 
driven before the advancing ice, or borne on its surface, and that now 
run along or across the valleys like ramparts ; and other equally striking 
and conclusive phenomena. Thus we arrive at the undeniable fact, that 
the British Isles once exhibited in successive periods all the ice-features 
of Greenland, Norway, and the Alps. 
The Rocks as Related to Time. 
The Length of Geological Periods, or Geological Time—In studying 
Geology, it is necessary to have an accurate notion regarding the nature 
of the periods spoken of. It is to be strictly observed that in Geology, 
time cannot be measured by years. When we examine any stratum of 
rock, with all its enclosed organisms, it is natural to inquire how long this 
mass of rock took to be deposited. We can judge of this only in the 
following way. From observation of river-action as at present exhibited, 
we see with what extreme slowness rock-masses are worn down into sand ; 
how a thousand years make an almost imperceptible change on a boulder, 
and even on the gravel by the shore. Yet we know that the sandstone 
before us, often hundreds of feet in thickness, is composed of grains of 
tock ground down by water-action, transported by rivers to the sea- 
bottom, and deposited there till other strata were heaped upon it; and 
that in after-ages the grains united, and were hardened by pressure into 
the rock we see. What incalculable ages, therefore, must this sandstone 
bed have taken to be thus formed! The more we think of these slow- 
working causes, the more are we astonished at the enormous periods of 
time that must have elapsed before the formation of even the thinnest 
‘layer of rock. Geological periods, therefore, are quite indefinite in the 
matter of yedrs; but from various considerations, we can arrive at certain 
very definite conceptions regarding the length of time required for the 
formation of the various rock-systems. This inability to assert a definite 
number of years in regard to any formation, is no defect in the science, 
for the knowledge of this would add not one item to the conception we 
