Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/126

 crystals from Tol Cam mine are generally of great length and remarkable transparency, those from Tin Croft are more generally opake; but the long and slender crystals from the former mine rarely suffer interruption from each other, being generally deposited at right angles, and thereby shewing a constant tendency to assume a quadrilateral figure.

There are in my possession about 55 specimens of the oxyd of uranium from the various mines in Cornwall above cited, and upwards of 200 detached portions, each having one or more well defined crystals on them and placed on pieces of wax; from these the accompanying drawings were made. Pl. 5, 6, and 7.

Varieties of the Primitive Crystal.

It has been already noticed that the primitive crystal of the oxyd of uranium is considered to be a tetrahedral prism, with square bases. I have never observed any crystal, exhibiting the primitive planes only, of a greater elevation than that described by fig. 1. Fig. 4 represents an elongated crystal: this elongation is so considerable on several crystals from Tol Carn mine that he length exceeds the breadth many times.



First Modification.

The first modification is the result of a decrease along the lateral edges of the primitive prism, by which each is replaced by a quadrangular plane. The crystals represented by figs. 6, 7, and 8, are the most common of the whole series.