Dr. M'^Culloch's Additional Remarks on Glen Tilt. 69 



eringof the rock, which leaves it projecting on the surface/where it is scarcely 

 discoverable in the fresh fracture. There are few minerals which, with a 

 steady geometric form, present so wide a range of aspect and composition. 



Mica. — Independently of the common scaly form under which this sub- 

 stance occurs, in mixture with the ordinary rocks, I have found it, in the hills 

 so often mentioned, under three distinct forms of crystallization, all of them 

 more or less interesting, particularly as it is not often met with in this country 

 in a detached and crystallized state. 



It occurs imbedded in porphyry in the form of regular hexagonal prisms, 

 about the tenth of an inch both in diameter and length : in all the specimens 

 which I observed it is of a black colour. It is not very unusual to meet with 

 it in porphyries, but it is rarely seen of so regular and perfect a form. Per- 

 haps the most interesting circumstance attendant on this mica is the uniform 

 direction of the crystals, the flat surfaces of the prisms all lying in parallel 

 planes : — an occurrence which I have on other occasions noticed, both in or- 

 dinary porphyry veins and in veins of trap belonging to the most recent for- 

 mations of that substance. A very important geological inference may be 

 drawn from it respecting the disposition of the mica in those rocks which are 

 supposed to owe their laminar structure to stratification. It is evident that this 

 parallel disposition in gneiss and in micaceous schist, is not sufficient to prove 

 that the mica has been mechanically deposited from a state of suspension ; 

 since, in rocks which from their venous form have unquestionably not been 

 deposited in this manner, it equally liolds a parallel arrangement. I may add 

 that the parallelism of the mica in the porphyry veins, is conformable to the 

 fissile tendency of the including rock, which, in all the cases that I have yet 

 observed, is also parallel to the sides of the vein, even where that is at right 

 angles with the surrounding strata, so as to leave no uncertainty respecting its 

 truly venous character. There is no doubt that this universal parallelism, in 

 crystals so numerous and so widely separated from each other, must depend on 

 some common crystalline polarity ; although we may be unable to explain this 

 property, which is, however, no more obscure than every other phenomenon 

 connected with crystallization. I have formerly pointed out analogous ex- 

 amples ; and they are of frequent occurrence in cases where the phenomenon 

 appears to have escaped notice, of which one of the most remarkable is that 

 of graphic granite. In this it often happens that the same face of all the in- 

 terrupted crystals of felspar through an extensive vein, is placed in the same 

 direction : a fact easily examined in sunshine by taking reflections on various 

 fractured points of the whole. I shall not here pursue this subject further ; 



