Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/167

 Rh tors seem to have undergone great changes by the lapse of time, and they were probably higher at a former period than they now are.

The grauwacke covers the lower part of Carn Marth, and rises exactly to the height of five hundred and forty-three feet, which is about two-thirds of that of the mountain.

According to the mean of nine barometrical observations, the height of Redruth at Gray's Hotel, in the middle of the town, is four hundred and fourteen feet above the level of the sea; about two hundred paces beyond the first milestone from Redruth to Truro, the grauwacke disappears, and is superseded by a ridge of granite which continues exactly to the end of the first mile; there the land lowers, and we re-enter the grauwacke, on which formation Scorrier House stands.

Porth Towan is another place on the Bristol Channel, four miles from Scorrier House, where the direction and inclination of the strata of grauwacke may be well observed. The cliff`s are high and rather abrupt, and the rock has been very much excavated at the bottom by the action of the waters. Quartz abounds in it.

I observed that the grauwacke assumed a more slaty structure as it approached the sea from Scorrier House. The sands at Porth Towan extend pretty far into the interior of the country.

St. Agnes's Beacon is an insulated eminence of a pyramidal form, situated N.E. of Porth Towan, a short way in the interior; it has nearly the same degree of inclination on all sides, and is quite covered with debris. It is entirely composed of grauwacke, though six hundred and sixty-four feet above the level of the sea, and in