THE FOSSILIFEROUS ROCKS. 



27 



thereby. Thus, in Hmestones which have been greatly altered 

 or " metamorphosed'' by the combined action of heat and pres- 



Fig. II. — Section of Carboniferous 

 Limestone from Spergen Hill, Indiana, 

 U.S., showing numerous large-sized 

 Foramiiiifera {Endothyra) and a few 

 oolitic grains ; magnified. (Original.) 



Fig. 12. — Section of Coniston Limestone 

 (Lower Silurian) from Keisley, Westmore- 

 land; magnified. The matrix is very coarse- 

 ly cr^^stalline, and the included organic re- 

 mains are 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 

 aggregate 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. Magnesian limestone- 

 ox 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 



