28 PRINCIPLES OF PALiEONTOLOGY. 



carbonate of magnesia, and are highly crystalline. The ordi- 

 nary magnesian limestones (such as those of Durham in the 

 Permian series, and the Guelph Limestones of North America 

 in the Silurian series) are generally of a yellowish, buff, or 

 brown colour, with a crystalline or pearly aspect, effervescing 

 with acid much less freely than ordinary limestone, exhibiting 

 numerous cavities from which fossils have been dissolved out, 

 and often assuming the most varied and singular forms in con- 

 sequence of what is called " concretionary action." Examina- 

 tion with the microscope shows that these limestones are 

 composed of an aggregate of minute but perfectly distinct 

 crystals, but that minute organisms of different kinds, or 

 fragments of larger fossils, are often present as well. Other 

 magnesian limestones, again, exhibit no striking external pecu- 

 liarities by which the presence of magnesia would be readily 

 recognised, and though the base of the rock is crystalline, they 

 are replete with the remains of organised beings. Thus many 

 of the magnesian limestones of the Carboniferous series of the 

 North of England are very like ordinary limestone to look at, 

 though effervescing less freely with acids, and the microscope 

 proves them to be charged with the remains of Forantinifera 

 and other minute organisms. 



Marbles are of various kinds, all limestones which are suffi- 

 ciently hard and compact to take a high polish going by this 

 name. Statuary marble, and most of the celebrated foreign 

 marbles, are " metamorphic " rocks, of a highly crystalline 

 nature, and having all traces of their primitive organic struc- 

 ture obliterated. Many other marbles, however, differ from 

 ordinary limestone simply in the matter of density. Thus, 

 many marbles (such as Derbyshire marble) are simply " cri- 

 noidal limestones" (fig. 9); whilst various other British 

 marbles exhibit innumerable organic remains under the mi- 

 croscope. Black marbles owe their colour to the presence of 

 very minute particles of carbonaceous matter, in some cases 

 at any rate; and they may either be metamorphic, or they 

 may be charged with minute fossils such as Foraminifera (e.g., 

 the black limestones of Ireland, and the black marble of Dent, 

 in Yorkshire). 



''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 maybe in almost immediate 

 contact with one another, or may be cemented together by a 



