Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/125

 as it depends, in the first place on the precise admeasurements of certain angles, which no crystal I have hitherto seen has enabled me to make, and in the second place, on calculations founded thereon, for which I possess not the requisite qualifications, I am compelled simply to state the opinions of these crystallographers, and shall be amply gratified if, in their estimation, what is now offered in regard to cleavages not before obtained, and modifications not before noticed, shall tend to throw any light on this important part of the subject.

The occasional notice of what appear to be perfect cubes, added to the certainty that cleavages are practicable parallel with every plane of that solid, at first tempted me to presume that it ought to be considered as the primitive form; but the consideration of some circumstances connected with the crystallization of this substance, induced me wholly to abandon the idea. The inspection of the annexed drawings will evince that, of the five modifications described, the first alone is compatible with the cube.

All the crystals represented in the accompanying series which are so remarkably flat as to have the appearance of mere laminæ, are from Tol Carn mine, which yielded only such as are of that description, while on the other hand, the greater part of those from Tin Croft mine nearly approach in height the dimension of the cube, or exceed it. A considerable proportion of such as are rather less than that dimension exhibit no striæ on the lateral planes, which are very splendent: many of those which are deeply striated on these planes are iridescent on the surface. The