Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/120



II. On the Oxyd of Uranium, the production of Cornwall, together with a description and series of its Crystalline forms.

By, Member of the Geological Society.

Read February 17th, 1815.

only mine in Cornwall which until within the last few years was known to have yielded the oxyd of uranium, was that called Carharack, which was situated about two miles nearly south of St. Die. The crystals on a specimen from that mine in my possession are tabular, of a green colour and transparent, except such of them as are partially or wholly coated by a deposition of an ochreous substance, similar to that termed gossan by the miner. This substance also is interposed between an aggregated quartz tinged with iron, and the crystals; some of which are imbedded in it. It may therefore be termed, in regard to this specimen, the matrix of the crystals. On many of them have been deposited numerous minute cubes of a light green colour; which, as there is a considerable deposition of cubic arseniated iron in a cavity of the same specimen, I consider to be that substance rather than the oxyd of uranium: for though the latter sometimes takes a form so nearly approaching to the cube as that the eye cannot perceive any difference, yet such instances are certainly not very common.

In 1805, I noticed some crystals of the oxyd of uranium on the refuse heaps of Tin Croft mine, which is at the foot of a granitic hill called Carnbrae near Redruth; the veins of that mine run