CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 



19 



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

 sure, all traces of organic remains become annihilated, and 

 the rock -becomes completely crystalline throughout. This, 



Fig. 4. Section of Carboniferous Lime- 

 stone from Spergen Hill, Indiana, U.S., 

 showing numerous large-sized Foraminifera 

 (Kn/kithym) and a few oolitic grains mag- 

 nified. (Original.) 



Fig. 5. Section of Coniston Limestone 

 (Lower Silurian) from Keisley, Westmor- 

 land ; magnified. The matrix is very coarse- 

 ly crystalline, and the included organic re- 

 mains are chiefly stems of Crinoids. (Ori- 

 ginal.) 



for example, is the case with the ordinary white " stat- 

 uary 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 neces- 

 sarily 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, Crinoids, 

 Forarninifers, and Mollusca. 



As is the case with the ordinary limestones and marbles, 

 so also the various kinds of magnesian limestone and dolomite 



