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slates and shales we can in some measure trace, though even 
here there are links missing in our knowledge. We know but 
little, for example, of the origin of the amorphous silica, which 
cements the grains of sand together to form sandstone 1 have 
never been able to find, however, even an attempt to trace 
quantitatives, the relations between the constituents ot the 
primary rocks and those of the different locks of eac succeet in 0 
period, and till we have done this, we cannot claim to have 
certainly traced their origin. But besides the clays and sands 
of which we have spoken, there is a no less important class ot 
rocks, the origin of which is a most difficult question. e can 
actually watch the formation of clay; but what about limestones . 
The analytical process which can extract a pure marble rora . 
diluvial mass is certainly a most remarkable one. The chemist 
is here at fault. The methods he would use, however effectual 
in the laboratory, are certainly not those of nature, and we must 
look elsewhere for the explanation, lhere is one agent, no 
doubt, that we find in nature which can effect this separation, 
and as far as I know only one— that is, the life of the ower 
animals. By that mysterious power of which we know so little, 
that we call life, a zoophyte can extract the dissolved lime from 
water and give us a coral of pure carbonate of lime ; and the 
combined labours of countless myriads of globigennue have 
sufficed to build up the chalk to its vast thickness No branch 
of investigation has given more interesting results than those 
of the deep-sea soundings of the Lkallenyer, showing, as tie} 
do, that the process of chalk-formation is now going on, in the 
same manner that had been determined from the examination 
of the chalk of past ages. It may be that chalk and coral are 
examples of the mode of formation, which alone in the past 
a«-es of the world has produced the limestone formations, and the 
crystalline form induced afterwards by subsequent changes ; at 
anv rate, we have no certain knowledge of any mode by winch 
carbonate of lime is separated in a pure state in nature excep 
by the operation of animal life. 
19. In the chalk formation, as we all know, occur the flints, 
which again in later formations supply the material for gravel, 
when the chalk has been washed away; a process familiar 
enough to any one who has walked over the shingle at the loot 
of a°chalk cliff'. Much as we know about the formation of 
chalk we have yet learnt very little of the origin ot the 
companion flint. ' It has been attributed to marine infusoria 
and sponges, but this is, as yet, little more than a guess, as 
