28 



PRINCIPLES OF PALAEONTOLOGY, 



In other cases, the matrix is more or less crystalline, and when 

 this crystallization has been carried to a great extent, the original 

 organic nature of the rock may be greatly or completely obscured 

 thereby. Thus, in limestones which have been greatly altered 

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



Mg. 11. Section of Carboniferous 

 Limestone from Spergen Hill, Indiana. 

 U. S., showing numerous large-sized 

 Foraminifera (Endothyra) and a few 

 oolitic grains ; magnified. (Original.) 



Fig.12. Section of Ccniston Limestone 

 (Lower Silurian) from Keisley, West- 

 moreland; magnified. The matrix is 

 very coarsely crystalline, and the in- 

 cluded organic remains are chiefly stems 

 of Crinoids. (Original.) 



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 high 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 generalization 

 that limestones are fundamentally the product of the operation 

 of living beings. This fact remains certain; and when we con- 

 sider 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. 



