Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/22

 10 also be traced gradations into mere quartz; and I further observed some varieties consisting of wavy mixtures of quartz and hornblende only.

This stone has a considerable tendency to decomposition, the felspar and mica being both very ferruginous. It is consequently found in all states, from that of a friable rock, to a gravely clay, and finally to a perfect soil; constituting the gravelly or sandy loans which predominates through the Island.

No where is it more easy to remark the process by which in nature rocks are converted into earth; and as in this case by the action of an oxide of iron. Nests of yellow mica which seem to have arisen from the decomposed rock, are found in many places.

This stone is used for rough masonry.

A ledge of rocks called the Hanois extends from the westernmost point of the island, and from its apparent geographical continuity is probably of the same structure. Against this point the whole strength of the western ocean is first directed. And it is from hence I doubt not, that a large ridge of rounded masses of stone has been rolled, so as to form a natural barrier near Rocquaine.

In quitting the elevated part of the island, and with it the southern shore, the gneiss disappears; and its place is supplied by other granitic formations. Besides the trap and trap porphyry which I remarked at Rocquaine, there are masses of micaceous schist, having the appearance of veins; and a stratum of argillaceous schist may also be observed at the lower parts of the bay, incumbent on the granitic foundation.

At l'Erée and Lihou the rocks are composed of quartz and felspar, the foliated texture having disappeared. A granitel is thus formed, which in some places receiving an addition of hornblende, passes into sienite. This is traversed here and there by veins of the same