THE STRUCTURE AND ORIGIN OF LIMESTONES. 
137 
organisms living at each period, yet it must also have depended 
on the accompanying mechanical and chemical conditions of 
the water in which the deposits were formed. The structure of 
each rock was, therefore, dependent on two most important cir- 
cumstances, and we need not be surprised to find the results so 
varied and characteristic. Passing upwards from the earlier 
rocks, we may often trace a gradual change, broken here and 
there by a complete contrast, which is in perfect agreement 
with results arrived at from a totally different class of facts. 
On the whole, this is perhaps the most important conclusion 
that we can at present draw from the subject before us. Possibly 
further research may teach us much more, since I am quite sure 
that much remains to be learned. In fact, long as I have studied 
these questions, and long as this address has been, I know 
quite enough of the facts to be convinced that it is only a sort of 
first attempt and rough sketch of a very wide and complex 
subject.” 
