Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/272

From Wikisource
Jump to navigation Jump to search
This page has been proofread, but needs to be validated.

still unsuccessful. In the direction of the lesser planes (P′ P′) and parallel with them, a cleavage is not only practicable, but may readily be obtained by the assistance of a sharp penknife, when the crystal is pressed on the fore finger beneath the thumb nail, which is the most effectual mode I have been able to find. The crystals are also divisible parallel with a section passing along the elongated summit and down the centers of the planes P′ P′ of a crystal formed like fig. 10. The search for natural joints in any other direction was fruitless.

If therefore we divide an elongated crystal (fig. 10.) in the direction of the dotted lines a b c d e and b c d, being parallel sections in the direction of its natural joints, we shall obtain a solid represented by fig. 12, which occurs in nature, and greatly resembles some crystals of the sulphate of barytes. If then this solid be cleaved parallel with the planes P′ P′, we shall obtain a nucleus similar in form to the dotted lines within it, and of course to fig. 13, which, though not in the same position, resembles in form, but not in measurement, the primitive crystal of the sulphate of barytes (fig. 2.); it is a right prism with rhombic terminations. Of these solids obtained from amorphous specimens of the sulphate of lead, I possess several, and am led to the conclusion that if we are to depend on the cleavage of minerals for a knowledge of the forms of their primitive crystals, this solid is that of the sulphate of lead.

In my collection there is an amorphous specimen from the Lead Hills, exhibiting natural joints parallel with all the planes of a right prism with rhombic terminations. It is covered on one of its larger sides by long and nearly flat crystals with diedral terminations, lying on the mass with the terminations parallel with the natural joints observable in it; and there is a still more perfectly characterized specimen in the collection of Mrs. Lowry.