62 Dr. M'^Culloch's Addilional Remarks on Glen Tilt. 



distance from the granite is no test of its real one ; as the mass of granite may 

 here be subjacent to the schistose rocks at no great distance, although no 

 where coming to the surface. I may add also, that minute ramifications of 

 granite are found traversing the schistose rocks, even in the bed of the Bana- 

 vie, within the grounds of Athol House. 



Trap Veins. — One of these was noticed in the original communication, 

 already referred to, and I have since observed another at Gow's Bridge, 

 during a low state of the waters of the river, which was not visible on former 

 occasions. Its course is nearly conformable to the direction of the strata, 

 which however it also intersects ; and it presents no peculiarities either in its 

 structure or its junctions. In many parts of Scotland such veins would be 

 unworthy of notice ; but they are deserving of record in a place where they 

 are of very rare occurrence ; which is the case wherever we recede far from 

 the great overlying masses which abound in the middle, and on the western 

 shores of this country. 



Limestone. — Among the micaceous schist and gneiss which form the 

 northern boundary of the Tilt, and not far from its junction with the Garry, 

 there occurs a remarkable variety of primary limestone. It is of a laminated 

 structure, each lamina of calcareous matter being separated from the next by 

 one of mica. The calcareous laminae rarely exceed the twentieth of an inch 

 in thickness, and are often much less. The stone is readily fissile in the 

 direction of the micaceous lamina ; and as the calcareous one is only visible 

 on the edge of the fractured specimen, on account of the continuity of the 

 micaceous surface, it is easily mistaken for micaceous schist. The calcareous 

 part of the stone is of a pale blueish-gray, the micaceous of a pale brown; and, 

 as the laminae are perfectly flat and parallel, a section at right angles to them 

 presents an appearance as regular as a succession of ruled lines. 



The yellow marble, described at page 295 of the Memoir above mentioned, 

 is of an uniform ochre colour only in one point. A more complete examina- 

 tion of the beds has shown that the predominant mass is of a white colour, 

 intersected by numerous reticulating veins of pale ochry yellow. At the 

 edges of the bed, where it comes into contact with the schistose rock, it 

 assumes a grayish colour and loses its beauty ; while it acquires great hard- 

 ness and an argillaceous aspect. Marbles of this colour are much esteemed 

 by architects on account of the warmth of their tints and the tranquiUity of 

 their eifect in interior decoration. 



The pink marble mentioned at page 306 has since been found in its natural 



