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 those of the trap islands 
of the Western coast." The same writer mentions 
that in Shetland, a granite composed of hornblende, 
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 
