THE FOSSILIFEROUS ROCKS. 29 



Amongst the numerous varieties of limestone, a few are of 

 such interest as to deserve a brief notice. Magnesia limestone 

 or 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 

 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 color, 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. " Examination 

 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 peculiarities by which the 

 presence of magnesia would be readily recognized, and though 

 the base of the rock is crystalline, they are replete with the 

 remains of organized beings. Thus many of the magnesian 

 limestones of the Carboniferous series of the North of Eng- 

 land are very like ordinary limestone to look at, through 

 effervescing less freely with acids, and the microscope proves 

 them to be charged with remains of Foraminifera 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 color to the presence of 

 very minute particles of carbonaceous matter, in some cases 

 it 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). 



