Page:A Treatise on Geology, volume 2.djvu/119

 CHAP. VII. veins; and this same country is broken into innumerable parts by metallic lodes, elvan courses, and other accompaniments of subterranean dislocations.

Most thankful, therefore, should geologists be, that further investigation of the facts, on which so many hands have been employed, has been performed by Mr. De la Beche, whose report, accompanying the geological survey of Devon and Cornwall, has now passed through the press.

From professor Sedgwick's description of the magnificent phenomena of granite veins at Trewavas Head, about two miles west of Forth Leven (Cambridge Philosophical Trans.), we extract the following notice:—

"On reaching the beach, we first found the killas rocks intersected by many contemporaneous veins of quartz. Not many feet farther west we were surprised to observe an appearance of alternation between the slate on which we were advancing, and several thin beds of granite. One more especially, which towards its southern extremity was lost under the waters, preserved its thickness and conformity to the laminæ of the schist for more than 100 feet. But its true nature was easily determined in the other direction; for it gave out several smaller veins, then cut obliquely through the laminæ of slate, and at length contracted its dimensions, started entirely from its previous direction, and ran in a flicker, ing line across the perpendicular cliffs. This vein is in no part more than two feet wide; yet it may be traced from the edge of the water to its termination in the cliff, nearly 400 feet.

"In the cliffs further west there are several granitic veins, which would be considered of no great interest if they had not been intersected by two other veins of different character, which must be classed either with the metalliferous lodes or the cross course of the country. One of them ranges nearly in the magnetic meridian, is about one foot and a half wide, and underlies east, two feet in a fathom. The other underlies in an opposite direction. They both contain quartz, oxide of