432 Dr. Mac Culloch on the Geology of 



of curved penicilliform groups of acicular crystals frequently an 

 inch in length, assuming an aspect of great singularity. In this 

 direction the schist is visible, and appears to form the largest part 

 of the stone, while in the cross fracture the lamellae of hornblende 

 alone being seen, the whole rock seems to consist of this mineral. 

 Occasionally the hornblende displays crystals disposed in so many 

 different ways, that the schist is discernible even in the cross fracture, 

 but this variety is the least common. 



Portsoy, 



The veins of granite found at Portsoy are well known, not only 

 for the peculiar character of their crystallization, called graphic, 

 but also because they contain crystals of schorl (tourmaline) of 

 great magnitude. The singular disposition, and mutual relations 

 of the crystallized substances which form this compound rock, are 

 known to have afforded Dr. Hutton an argument for its igneous 

 origin, and its peculiar character has been supposed to arise from a 

 simultaneous, or nearly simultaneous crystallization of the several 

 substances contained in it. The specimens which I have to 

 enumerate, are such as not only throw considerable doubt on this 

 explanation, but are in fact, sufficient to prove a sequence of 

 epochas even in this limited space, and to show that the compound 

 rock in question has been formed by successive operations, the 

 nature of which however I fear we shall not easily determine. 

 I would entreat the pardon of the Society for occupying so much 

 of its time on the fracture of a schorl crystal, were I not persuaded 

 that much light must at some period inevitably be thrown on the 

 greater geological phenomena, by considering the chemical and 

 mechanical relations existing among the smaller portions which 



