Ch. V.] PRIMARY SCHISTOSE ROCKS. 71 



with a paler tone of the same colour. These generally lie 

 near the gneiss; the interior gradually assuming a more 

 massive form, and displaying, at length, a fracture from 

 which all appearance of a foliated tendency has vanished. 

 Talc-slate is also occasionally found at the limit of the gneiss, 

 mixed more or less with quartz of a very greasy aspect. In 

 the interior of the latter rock, the bed of which, at its thickest 

 part, may be about a hundred yards, a body of potstone is 

 found ; the harder serpentine passing into it by gentle de- 

 grees. This serpentine is of a dark green, and somewhat 

 translucent on the edges, like wax : it contains veins of dark 

 green, and of pure white steatite, sometimes fibrous, together 

 with splendent veins of greenish asbestos.* 



The gneiss of the Western Islands, like that of the main- 

 land of Scotland, contains limestone. In Tirey, this cal- 

 careous rock occurs in several places. Not far from Balphe~ 

 trish, it is of a reddish hue ; varying from a high flesh-colour 

 to nearly white, and from a muddy crimson to a dull pur- 

 ple. It is of a fine splintery fracture and smooth grain. It 

 is an irregular mass or nodule, of about a hundred feet in 

 diameter, and is surrounded on all sides with gneiss. It is 

 not, therefore, a bed, as it is sometimes improperly so called, 

 but a nodule ; a form common to the greater number of lime- 

 stones found in gneiss and in mica-slate. It is not capable 

 of being raised in parallel- sided masses ; yet, after exposure 

 to weather, it splits into laminae with great facility. It con- 

 tains concretions of hornblende, but more commonly of 

 augite ; the green colour of which contrasts well with the red 

 colour of the ground. The contact of this rock with the 

 gneiss is well defined ; and of all the component parts of the 

 latter, quartz is most generally in union with the limestone; 

 and is, probably, intimately combined throughout its mass, 

 as indicated by the great hardness of this marble. Lumps of 

 granite or gneiss are occasionally imbedded in this lime- 



* Western Islands of Scotland, vol. i, p, 168. 

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