THE FOSSILIFEROUS ROCKS. 27 
thereby. Thus, in limestones which have been greatly altered 
or “metamorphosed” by the combined action of heat and pres- 
Fig. 11.—Section of Carboniferous Fig. 12.—Section of Coniston Limestone 
Limestone from Spergen Hill, Indiana, (Lower Silurian) from Keisley, Westmore- 
U.S., showing numerous large-sized land; magnified. The matrix 1s very coarse- 
Foraminifera (Endothyra) and a few ly crystalline, and the included organic re- 
oolitic grains ; magnified. (Original.) meee chiefly stems of Crinoids. (Ori- 
ginal. 
sure, all traces of organic remains become annihilated, and the 
rock becomes completely crystalline throughout. This, for 
example, is the case with the ordinary white “statuary marble,” 
slices of which exhibit under the microscope nothing but an 
ageregate of beautifully transparent crystals of carbonate of 
lime, without the smallest traces of fossils. ‘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 differ- 
ent 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. A/agnestan limestone- 
or dolomite, differs from ordinary limestone in containing a cer- 
tain proportion of carbonate of magnesia along with the carbon, 
ate of lime. The typical dolomites contain a large proportion of 
