74 



GRANITE. 



ner into a basalt, and at last into a soft clay stone, 

 with a schistose (slaty) tendency on exposure, in 

 no respect differing from chose of the trap islands 

 of the Western coast." The same writer meniions 

 that in Shetland, a granite composed of hornbUnde, 

 mica, feldspar, and quartz, passes in an equally per- 

 fect manner into basalt. 



Granite occurs in masses of vast thickness, which 

 are commonly divided by fissures into blocks that 

 approach to a rhomboidal shape ; and sometimes, 

 in mountains, we see it assume a columnar struc- 

 ture. Granite forms sometimes extensive ranges of 

 mountains, whose aspect varies extremely. Where 

 the beds are nearly horizontal, or where the gran- 

 ite is soft and disintegrating, the summits are round- 

 ed, heavy, and unpicturesque ; but where it is hard, 

 and the beds are nearly vertical and have a laminar 

 structure, it forms lofty peaks, that shoot their nee- 

 dle-shaped spires into the sky ; as in the vicinity of 

 Mont Blanc, where the Aiguille de Dree rises above 

 its base nearly to a point in one solid shaft more 

 than 4000 feet high. In the Andes the granite rises 

 to an elevation of about 12,000 feet ; the summits 

 of which are covered by basalt, porphyry, and lava 

 ejected from the numerous volcanoes, which now 

 exist or formerly existed among them. The sum- 

 mit of Chimborazo, which is 21,440 feet in height, 

 is a vast cone or crater composed of volcanic pro- 

 ductions covered by snow. The general arrange- 

 ment of the Andes," says Humboldt, "consists of 

 granite, gneiss, mica, and clay slate, on which are 

 frequently laid porphyry and basalt, arranged in the 

 form of regular and immense columns, w^hich strike 

 the eye of the traveller like the ruins of enormous 

 castles lifted into the sky." Where hills are form- 

 ed of granite, they are, for the most part, of a pecu- 

 liar rounded form, wholly clad with a scanty vege- 

 tation. The surface of the rock is generally in a 

 crumbling state, and the hills are often surmounted 



