Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/454

 of the mountain, and primitive slate the sides. The contacts are as sharp as possible, without the least of one rock graduating into the other, and in all cases the granite is continued from the great mass in veins through the slate, but never the contrary. Masses of slate often occur, like islands floating in, and surrounded by the granite of the veins. Mr. Playfair, who was with Dr. Mac Donnell, remarked that the schist, which lies upon and near the granite, has a much greater number of fissures than that which is a mile distant. The granite veins generally terminate in fine threads.

A letter from George Cumberland, Esq. of Bristol, was read, giving an account of a trap rock that had been discovered at Micklewood, in Gloucestershire. It occurs to the east of the road going from Bristol, within two miles of Frampton, on an estate belonging to Lord Berkeley, and is known by the name of the Old Raock. The mass rises perpendicularly to the height of about 30 feet, is less than 300 yards wide, and extends in the other direction about a quarter of a mile. The same rock is found again to the north-east of the first mentioned place, dipping to the east beneath the surface.

The Micklewood rock has an amygdaloidal character, containing plain or striped chalcedonies, and numerous fungi form or irregularly cylindrical masses, composed of iron spar. Those masses are often found two feet in length; the chalcedonies vary from one to twenty inches in diameter, and are nearly all of the same shape, convex above, and concave beneath.