Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/97

Rh Parts of this rock resemble Fullers' earth; but from the occasionally green colour and the peculiar direction of the natural rifts, giving it a tendency to separate into rhomboidal or into wedge-shaped fragments, it possesses a characteristic mark which serves to connect it with the prevailing rock of the neighbourhood.

Many parts of this rock easily crumble into the state of an earthy gravel, and are commonly used as a substitute for common gravel in and about St. David's.

I shall readily be excused for mentioning here, that there is in the Ashmole Museum a specimen from Jersey so very like in its general character to the part of the St. David's rock now under consideration, that even an experienced eye might be deceived as to its separate identity: and it adds to the interest of the comparison of the specimens in question, that they both occur amongst a suite of rocks composed principally of hornblende and felspar, and are both used for the same economical purpose.

To the south west of St. David's are the remains of an old entrenchment, situated near the edge of the adjacent cliff; and from the further extremity of the entrenchment the cliffs run out at right angles to the general bearing of this part of the coast, forming a tongue of land which projects into the sea. In this projecting point a natural arch exists, which has been probably excavated by the gradual washing away of part of the rock; presenting an appearance somewhat like that represented in the third of Dr. Mac Culloch's plates, in the first volume of the Society's Transactions.

Near the extremity of this tongue of land is a vein of clay