GRANITE. 121 



Clyde, with others in various parts of Ireland. The 

 Devonian group is developed in Devonshire, Herefordshire, 

 Monmouthshire, and Shropshire; and in Caithness, Cro- 

 marty, and other parts of Scotland. The Silurian group 

 occurs in Gloucestershire, Worcestershire, Staffordshire, 

 Herefordshire, Shropshire, Radnorshire, Montgomeryshire, 

 Caermarthenshire, Brecknockshire, Pembrokeshire, and Mon- 

 mouthshire. The Cambrian and Cumbrian systems consist 

 of masses of sub-crystalline and slaty rocks developed in 

 Cumberland and in "Wales, and the mica-schist, gneiss 

 and granite formations occur in the Highlands and Western 

 Isles of Scotland, and in Ireland ; the whole series forming, 

 with one or two trifling omissions, a complete epitome 

 of the physical geology of the earth, in the comparatively 

 limited extent of a few hundred miles. 



PLUTONIC BOCKS. GRANITE. Granite is a compound, 

 crystalline rock, composed of crystals of mica, quartz, and 

 feldspar, aggregated together, and separable by mechanical 

 means. The mineral which generally predominates is feld- 

 spar ; it is most commonly dark red, or white ; quartz is the 

 prevailing substance next to feldspar, and has a propor- 

 tionate share in determining the colour of the mass ; it is 

 usually white or grey. Mica, when black, imparts that hue 

 to the rock, but when, as frequently occurs, it is brown or 

 white, it gives those tints to the substance ; while the admix- 

 ture of hornblende, which is dark green, or black, produces a 

 rock varying between these shades. It is evident, therefore, 

 that a substance, consisting of various minerals, themselves of 

 different hues, and mingled in different proportions, must be 

 correspondingly diversified in tint, and thus we have granites 

 of almost all hues, red, white, black, and grey : the most pre- 

 valent being white, which is the tint presented by that of 

 Cornwall ; or red, which is the colour chiefly exhibited by the 

 Scottish varieties. A striking example of each kind is 

 exhibited in immediate juxta-position in Waterloo-bridge, 

 where the coping-stone is of the red, while the balustrades 

 are of the white variety. There are other substances which 

 enter into the composition of granite, as chlorite, talc, acti- 

 nolite, steatite, clevelandite, compact feldspar ; but specimens 

 of this nature are comparatively rare. 



Granite rises to the highest elevations and forms the 



