88 Dr. Mac Culloch's Sketch of the 



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 eft'ectually excluded from them. This mealy variety 

 appears in three different forms : in the first it is disposed in a ra- 

 diated 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 mi- 

 nuteness, 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 re- 

 specting 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 geo- 

 metrical 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 



