PRINCIPLES OF PALEONTOLOGY. 



" Oolitic " limestones, or " oolites, " as they are often called, 

 are of interest both to the palaeontologist and geologist. The 

 peculiar structure to which they owe their name is that the rock 

 is more or less entirely composed of spheroidal or oval grains, 

 which vary in size from the head of a small pin or less up to the 

 size of a pea, and which may be in almost immediate contact with 

 one another, or may be cemented together by a more or less 

 abundant calcareous matrix. When the grains are pretty nearly 

 spherical and are in tolerably close contact, the rock looks very 

 like the roe of a fish, and the name of " oolite " or ' egg-stone " 

 is in allusion to this. When the grains are of the size of peas or 

 upwards, the rock is often called a "pisolite" (Lat. pisum, a 

 pea). Limestones having this peculiar structure are especially 

 abundant in the Jurassic formation, which is often called the 

 " Oolitic series " for this reason ; but essentially similar lime- 

 stones occur not uncommonly in the Silurian, Devonian, and 

 Carboniferous formations, and, indeed, in almost all rock groups 

 in which limestones are largely developed. Whatever may be 

 the age of the formation in which they occur, and whatever 

 may be the size of their component " eggs, " the structure of 

 oolitic limestones is fundamentally the same. All the ordinary 

 oolitic limestones, namely, consist of little spherical or ovoid 

 " concretions, " as they are termed, cemented together by a larger 

 or smaller amount of crystalline carbonate of lime, together, in 

 many instances, with numerous organic remains of different 

 kinds (fig. 13). When examined in polished slabs, or in thin sec- 

 tions prepared for the micro- 

 scope, each of these little con- 

 cretions is seen to consist of 

 numerous concentric coats of 

 carbonate of lime, which some- 

 times simply surround an imag- 

 inary center, but which, more 

 commonly, have been suc- 

 cessively deposited round 

 some foreign body, such as a 

 little crystal of quartz, a clus- 

 ter of sand-grains, or a minute 

 shell. In other cases, as in 

 some of the beds of the Car- 

 boniferous limestone in the 

 North of England, where the 

 limestone is highly " arenaceous, " there is a modification of the 



mg i3.-siice of oolitic limestone 

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