276 Dr. Mac Cullock on the Geology of Glen Tilt. 



unintelligible. J :Tiay only say that schist, granite, and quartz rock of 

 different aspects, with occasional masses of porphyry, succeed each 

 other so frequently that the whole might be considered by those who 

 are prepossessed respecting systems of alternation, as a demonstrable 

 instance of this disposition. I hope to give a more correct view of 

 their connections hereafter. On the left bank I may also generally 

 remark that a succession of dark blue limestone, rarely interrupted by 

 schist, quartz rock, and granite, is seen on the left bank from Poll 

 Tarff down to this place, and even beyond it as far as Gow's bridge. 

 An alluvial blank of about 800 yards follows this marble rock, 

 which is succeeded by 100 yards or thereabouts of granite ; and 

 after 150 y.irds more of an alluvial space, a large mass of rock 

 for the space of 200 yards and upwards is found traversing 

 and obstructing the stream, so as to form a cascade. This very 

 irregular mass of rock consists of granite and quartz rock 

 united. The quartz rock is of a peculiar aspect, being of a bluish 

 colour, very uniform and compact and approaching near to 

 common quartz in character, but still shewing evident marks of 

 foliation. It is disturbed and intersected by the granite in a way 

 which I need not now repeat, A thin stripe of limestone and schist 

 may also be seen interwoven in the granite. Below the cascade 

 there is once more a blank alluvial space of 100 yards, which is 

 succeeded by quartz rock for about the same space ; immediately 

 after which occur about fifteen yards of a yellowish and greyish lime- 

 stone or marble, very hard, and disturbed and traversed by granite 

 in the way already described. This is followed by a series of lime- 

 stone beds for about 200 yards, the first of which is blue and the 

 last five or six of a yellowish aspect : these are almost the first 

 limestone beds observed in descending the river which have the 

 same general dip and position with the beds of the mountain on the 



