kind, where neither air nor water can have access, and where they are accompanied by crystals of absolute transparency: they are also found intermixed with and investing solid nodules of the toughest varieties, deeply imbedded in large masses where the elements are effectually excluded from them. This mealy variety appears in three different forms: in the first it is disposed in a radiated or rather in a ramose manner, in fine fibres possessing the peculiar lustre and softness of the finest white pulverulent talc. In a second case it forms distinct globular concretions of extreme minuteness, not to be discovered without the aid of the lens; and in a third instance, which I observed near Loch Eynort, a mass of globules of solid radiated mesotype, very much resembling some of the oolites, is intermixed throughout with farinaceous scales of the same substance, having the greasy aspect and lustre already described.
In speaking of this substance I have, according to common usage, ranked it with the mesotypes, it appearing to have been thus placed, partly because it is found associated with them, and partly because of the theoretical views which have been held respecting its origin. It will be for mineralogists of more authority to consider whether it does not deserve a separate place as a species: the question is evidently of a nature not to be determined by geometrical analysis, as far at least as the varieties already found extend; and the delicacy and uncertainty of unassisted chemical analysis in questions of this nature, are far too great to tempt us to seek a new place for it by this kind of investigation.
The last variety of mesotype exhibits a distinct crystallization; but crystals of tangible magnitude are so rare that I only procured one specimen in which the forms could be determined. They sometimes consist of a square prism, considerably elongated: in
