74 



TALCOUS SLATE. PRIMARY LIMESTONE. 



Gneiss and mica-slate, being nearly similar in their constituent 

 parts and geological position, most of the metallic ores and minerals 

 found in one rock, occur also in the other. Crystalline limestone, 

 hornblende, talc, and serpentine, more frequently, form beds in mica 

 slate than in gneiss. The waved structure is very common in mica 

 slate, and the beds are, often, most singularly bent and contorted. 



Talcous Slate and Chlorite Slate appear to be different modifica- 

 tions of the same mineral substances : in the former the structure is 

 laminated, in the latter it is minutely laminated or granular ; the pre- 

 vailing colour of both inclines to green. These rocks are soft and sa- 

 ponaceous to the touch, and sectile. Mica-slate appears to graduate 

 into talcous slate, particularly in the vicinity of Mont Blanc. In Cum- 

 berland and Scotland, talcous and chlorite slate pass into common roof 

 slate, and alternate with it : the change appears to be owing to a great- 

 er mixture of magnesian earth in talc slate, than in common slate. 

 Some varieties of chlorite slate are harder and darker, and approach 

 nearly to hornblende slate. The passage from talcous slate to ser- 

 pentine forms potstone. In primary mountains, talcous slate frequent- 

 ly occupies the place of mica-slate, and is, sometimes, confounded 

 with it; the two minerals, talc and mica, nearly resembling each oth- 

 er. See Chap. III. The large plates of mica, which are made to 

 supply the place of glass in some lanterns and in the slides for micro- 

 scopes, are always miscalled talc. Sometimes, mica-slate, from an 

 intermixture with talc, forms an intermediate rock, which partakes of 

 the characters of both rocks : such mica-slate has, generally, a green- 

 ish colour, and is softer than common mica-slate. 



Crystalline or Primary Limestone, of which statuary marble is a 

 variety, occurs, principally, forming beds in primary mountains. 

 Beds of this mineral occur rarely, in granite, more frequently, in 

 gneiss, but are most common in mica-slate, with which rock it is often 

 much intermixed, and often alternates with it. It is observed, that 

 the primary limestone in granite and gneiss, is coarser grained than 

 that in mica-slate or common slate. Primary limestone is, often 

 much intermixed with serpentine. When beds of primary limestone 

 occur of considerable thickness, they sometimes contain veins of me- 

 tallic ores. 



Crystalline or primary limestone, when pure, is composed of cal- 

 careous earth, which scarcely exists, as a component part of granite, 

 gneiss or mica-slate. In primary mountains no organic remains are 

 found in the crystalline limestone; the structure is granular; the white 

 variety, known as statuary marble, resembles fine loaf-sugar, and is 

 imperfectly translucent ; hence, it has been called by the French 

 chaux carbonatee saccaroide. The color of primary limestone is, 

 sometimes, yellowish, greenish or inclining to red. From a mixture 

 of mica, it has, often, a slaty fracture and divides in plates. It may 

 be further deserving of notice, that primary limestone or statuary 

 marble, frequently, contains a considerable quantity of siliceous earth, 

 to which it owes its hardness and durability. 



