There are also other cases, where the limestone is not necessarily highly crystalline, and where no metamorphic action in the strict sense has taken place, in which, nevertheless, the microscope fails to reveal any evidence that the rock is organic.

Such cases are somewhat obscure, and doubtless depend on different causes in different instances; but they do not affect the important generalisation that limestones are fundamentally the product of the operation of living beings.

This fact remains certain; and when we consider the vast superficial extent occupied by calcareous deposits, and the enormous collective thickness of these, the mind cannot fail to be impressed with the immensity of the period demanded for the formation of these by the agency of such humble and often microscopic creatures as Corals, Sea-lilies, Foraminifers, and Shell-fish.
Amongst the numerous varieties of limestone, a few are of such interest as to deserve a brief notice.

_Magnesian limestone_ or _dolomite_, differs from ordinary limestone in containing a certain proportion of carbonate of magnesia along with the carbonate of lime.

The typical dolomites contain a large proportion of carbonate of magnesia, and are highly crystalline.