The history of this shore is the history of the whole coast as far as Mont Orgueil, where the granitic rock becomes more continuous. The castle itself is situated on a high promontory of it.
From Mont Orgueil to Rosel harbour, with the exception of a flat Shore in St. Catharine's bay, is a continuous cliff, affording no variety of structure, and not intersected by any remarkable veins.
But at Rosel a very singular rock commences, which appears to occupy the whole of Bouley Bay from Rosel to Belle-Hougue. At a distance it so much resembles the forms of the granites in Cornwall, that I should have set it down as such had I not examined it at hand.
It is an argillaceous breccia consisting of large and small scraps of schistus cemented by a basis of the same nature, but having entirely lost its tendency to a schistose fracture. I found some veins of a white hornstone porphyry which run in it. How it is connected with the granitic rock I could not find, but I traced it two miles into the country towards the church of St. Martin.
The whole remainder of the northern coast consists of rocks of sienite of various elevation, exhibiting generally broad and perpendicular faces to the sea. They are every where intersected by perpendicular veins running N and S, forming many remarkable caverns where they have been exposed to the action of the waves.
These veins wherever I saw them seemed to consist of granite, of which the felspar was commonly of a brick red colour. The sienite itself is in general white, consisting of variable mixtures of quartz, felspar, and hornblende, and varying therefore in colour. Most commonly the felspar is predominant.
In the rocks of Mont Mado the felspar is particularly abundant, and is of a flesh colour, constituting a very beautiful variety, which is also susceptible of a fine polish.
