Prehnite

Prehnite was said to have been first discovered by colonel Prehn, who brought it from the Cape of Good Hope; Haüy, however, says that citizen Rochon was beforehand with him.—It is now found in many parts of Scotland. The present specimen comes from Dumbarton, and, when examined, exhibits a curious arrangement of crystals, in groups, forming hemisphærically, and showing faces which are the edges of tabular crystals arranged somewhat in segments of circles, having the larger crystals in the centre; see the left-hand lower figure. These crystals seem to betray signs of eight faces besides the two broader ones, like the right-hand figures. The nucleus we could not positively determine; it seems, however, to be a nearly rectangular table, as expressed by the dotted lines: thus the corners are as it were unfinished, or truncated. The upper figure is of the natural size; the lower one, for the sake of explanation, is magnified. This fossil lines the cavities of a sort of Kragg of Kirwan. The same crystals are also found at Salisbury Craig, Edinburgh, The substance is sometimes found amorphous, as at King’s Park.