Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/14

 2 It will be seen that they are chiefly formed of granitic rocks. The Islands of Chozè, which lie deeper in the Bay, are of similar formation, and I am informed that Mont St. Michel is also a mass of granite. Excepting this, I have not been able to obtain any information with regard to the Coasts of Normandy or Brittany, from the Islands of Brehat to La Hogue. But from the Seven Islands to l'Isle de Siecle, including Morlaix and Treguier, I have had opportunities of ascertaining that granite is the predominant rock; and more extensive observation may possibly prove, that a chain of granitic rock extends from Cape La Hogue to Ushant, a line parallel to that granitic chain, which runs in a WSW direction, from Dartmoor to the Scilly Islands. This is rendered further probable, from the rockiness of the bottom of the sea, and the quartzose gravel and sand which are brought up by the sounding line.

The average depth of water in the neighbourhood of the islands is thirty-five fathoms; it is scarcely any where more than forty, and with the exception of a few shoals, the bottom is tolerably uniform.

Numerous rocks beset these coasts, some of which form large chains lying in an east and west position. A variety of currents is the consequence of the particular position of these islands in the Channel stream, and the intricacy and rapidity of them, tend to form a very difficult navigation, and a strong natural defence, to the islands.

The tides on these shores rise to a considerable height, though not nearly equal to their elevation in the bottom of the bay, where the check to the Atlantic wave is greatest. The height which they attain is from thirty to forty feet: at Guernsey it is thirty-two feet. At St. Maloes it is said to exceed sixty.

The great wave which enters from the Atlantic, striking directly against the projecting Coast of Normandy, first fills the bay, and then continues its course along the islands, and round La Hogue, up