BASALT 



ROCKS. 



ranite Veins- In some parts of Britain, and also in other countries, granite vein* 

 j been observed proceeding from the mass of granite rock beneath, and traversing in all 

 3tions the superior and overlying strata. In Glen Tilt, and at Cape Wrath, Scotland, 

 spieiss and mica-schist are intersected with numerous granitic veins, the intrusion of 

 :h must have been therefore of later date than the rocks they traverse, 

 ic Igneous Bocks generally form the crests or elevated peaks of mountain suramits; 

 if not always entirely composing the upper limit, have by their action and elevating 

 ! on other strata, given an elevation and direction to many of the principal mountain 

 is, thereby producing one of the chief physical features of the earth's surface. It is to 

 sffects arising from the action of igneous rocks, or to their peculiar structural cha- 

 ir, that the picturesque features of much mountain scenery is due, as well shown in 

 >old and rugged outline of the naked and abrupt rocks, and the gradually tapering 

 s, called Aiguilles, in the Alps. The Caucasus and Himalayas present examples of the 

 itions of ancient disruption or subsequent weathering of the rocky mass. To this 

 r agency may be attributed the singular forms of some of the granite of Cornwall. 

 ic principal elevations of Devon and Cornwall, as the Brent Tor, Dartmoor, Exmoor, 

 are composed of grey coloured, coarse, and sometimes very porphyritic granite, in 

 h large crystals of felspar are imbedded. Specimens of this may be seen in the pave- 



of London Bridge. Granite is also found hi Cumberland and Westmoreland, and in 

 esea. It also occurs in Scotland, and is extensively quarried near Aberdeen. 

 Syenitic Granite forms the chief part of the Malvern Hills, and a similar rock occurs 



Barrow-on-soar, Leicestershire, where it is extensively quarried as a road stone, 

 ite forms the principal chains of Norway, Sweden, and Finland, portions of the Alps, 

 nees, and mountain chains of Bohemia, also of the Ural, Altai, and Himalaya ranges, 

 t occurs over extensive tracks in Africa, and South America. 



anite and the allied rocks are extensively used in the arts and manufactures; some of 

 olossal figures in the Egyptian saloon of the British Museum, afford examples of the 

 yenitic granite, basalt, and other igneous rocks. The granitic rocks are a source of 

 icr useful material for the manufacturer, the china clay is derived from the decompo- 



of the felspar, one of the materials of granite, thus producing a substance from which 

 ner varieties of china and porcelain are manufactured. 



POKPHYRITIC GRANITE. 



AIGUILLE DE DRU, Alps. 



fOLDS, 174, STRAND. 



