GEOGNOSY AND GEOLOGY. 105 
Order 9. Magnesian Limestones. 
The rocks constituting this order consist of magnesian matter, or of a 
combination of carbonate of lime with carbonate of magnesia. 
Magnesian limestone is the purer combination of the two above- 
mentioned substances, and is divided into dolomite and compact magnesian 
limestone. Dolomite bears the same relation to the other rocks of this 
order, that marble does to the limestones: the crystalline granular structure 
is characteristic of it. Its colors are exceedingly varied, as also its degree 
of hardness. White predominates; the tint may, however, be blue, grey, 
yellow, or ferruginous (from oxyde of iron). It abounds in foreign 
ingredients, on which account it is not unimportant to the mineralogist. 
The compact variety possesses a brittle, flat, conchoidal fracture, and is 
generally harder than compact limestone. 
Fetid Magnesian Limestone contains a portion of bitumen, which 
imparts a dark color, frequently modified by oxyde of iron. The crystalline 
granular, which is either scaly or an aggregation of magnesian rhom- 
bohedrons, has a rough appearance, and an iridescent lustre on the 
crystalline particles. Other varieties are the compact, breccious, cellular, 
porous, and earthy. 
Magnesian Marl. This is a very impure magnesia, containing, in 
addition to the usual magnesian combinations, carbonate of iron or 
manganese, alumina, and silex. The fracture is earthy and uneven. When 
fresh it is of a grey or bluish color. When exposed to the weather, the 
carbonates of iron and magnesia suffer decomposition, and hydrated oxydes 
of these metals are produced, the former penetrating the white rock, and 
coloring it rust or liver-brown, while the latter is separated in a dendritic 
form. 
Ferruginous Brown Lime. A mixture of magnesian matter with 
carbonates of iron and manganese. When fresh, it is yellowish or reddish- 
white; when weathered, ferruginous. Three varieties are distinguished : 
scaly, granular, and compact. 
Order 10. Gypsum Rocks. 
The rocks belonging under this head, of far less geognostical importance 
than the lime rocks, consist of sulphate of lime. There are two species 
which represent this order : 
Gypsum (pl. 36, fig. 12), or the hydrous sulphate of lime, occurring as 
spathic, scaly, granular, compact, and breccious. The compact is most 
abundant; the other varieties are found in it in greater or less abundance. 
The characteristic color is white. Bitumen, which frequently, penetrates 
the rock, produces a dark color, and at times beautiful markings. In less 
quantities it colors the gypsum blue. A very pure and compact variety of 
gypsum is known as alabaster. Spathic gypsum not unfrequently occurs in 
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