GEOLOGY. 



658 



compound. The elements of gneiss, however, show decided symptoms of being less 

 entire than in granite, having lost their asperity, had the sharp angles rounded or broken 

 off as if water-worn, and display, more or less, a laminated and stratified structure. 

 These circumstances mark the aqueous origin of the rock, which, though obscure in some 

 cases, so as to render it difficult to distinguish it from granite, are in general very well 

 developed. Three principal varieties have been noticed : 



Mica Schist. Gneiss. Granite. 



The granitic, closely resembling granite, the crystalline grain being large and distinct. 



The schistose, having a slaty appearance, and easily splitting into layers. 



The laminar, having the various minerals disposed in distinct alternating laminae, 

 which give a striped appearance to the rock. 



The last variety is represented above in contrast with granite. The laminis are 

 frequently undulating and contorted in a very high degree, indicating " a troubled 

 condition of the water from which the ingredients fell; or a source of agitation 

 in the still yielding sediment, which seems scarcely ever to have occurred among the 

 secondary and later strata. The only plausible explanation of this remarkable 

 circumstance which has occurred to us," says Mr. Phillips, " is the agitation of the sea or 

 the soft sediment on its bed by heat ; exactly as in the bottoms of steam-boilers, the 

 calcareous sediment is formed in irregular undulating laminas, which appear on a cross 

 section very similar to the flexures in the laminas of gneiss." Except in the variety 

 which closely resembles granite, gneiss is very distinctly stratified, as in the beds about 

 Loch Sunart in Argyleshire, the strata exhibiting considerable contortions and convolu 

 tions. The colour is usually of a greyish or reddish white, but has a darker tinge when 

 hornblende instead of mica enters into its composition. Almost all the metals are met 

 with in gneiss ; the Saxon, Bohemian, and Saltzburg mines are worked in this rock ; but 

 in Great Britain the metallic ores are not rich in such situations. At Strontian in the 

 Scotch highlands, a lead vein, occurring at the junction of the gneiss and the granite, was 

 formerly celebrated. 



Gneiss has a very wide geographical range, for hardly any considerable range of 

 mountains occurs without it. In England and Wales it is scarcely known at all, but in 

 Scotland it occupies an estimated area of 9600 square miles. The external aspect of the 

 country in which it predominates has been thus described : " Occupying large tracts 

 of the central highlands, the characters which gneiss impresses on the scenery are very 

 distinctly seen. It is the least picturesque and most monotonous of the primary rocks, 

 the hills being flat and shapeless, and their sides embossed with small round protuberances, 

 between which the water stagnates and moss accumulates. Most gneiss districts seem 

 but a repetition of these features on a great scale, the hills being seldom serrated in 

 outline, or broken into rocky cliffs ; whilst the valleys or straths are wide and flat, full 

 of small lakes or pools, and disfigured by brown heaths and dark morasses. It has 

 altogether the aspect of a land newly raised from the ocean, in which the rivers have not 

 had time to hollow out channels for themselves, or to complete its drainage. In a 

 word, all who are in quest of the picturesque should avoid the pure gneiss districts, as 

 the few spots worthy of notice will be found separated by long dreary uninteresting 

 tracks." One of these excepted spots, we may suppose to be Cape Wrath, the north- 



