Dr. Mac Culloch on the Geology ofGkn Tilt, 323 



marked by extensive ruin ; the bare schistose foundation has also 

 been in many places exposed. The alluvium in question consists 

 of the materials of the surrounding country, but all of them 

 water worn and mixed with sand and gravel as from considerable 

 attrition. In this, as in numerous other examples, there is no 

 assignable mode in which the materials could have been either 

 formed, or deposited, by the slow action of the existing streams, 

 while the total absence of a similar deposit in other parts of the 

 valley equally subjected to similar causes, bespeak a different 

 origin. The cause appears to have been of a diluvian nature, and 

 the determination of the deposit to this point, may perhaps be 

 found in the form of the valley at this place and the obstruction 

 which it has offered to a mass of matter impelled through the 

 upper parts where no such obstruction to its deposition existed. 

 Such an obstruction as that which I have suggested might formerly 

 perhaps have been rendered more complete by the continuity of 

 the strata beyond this point, which has since been gradually destroyed 

 by the slow action of the Tilt : in consequence of this the deposit of 

 alluvial matter has itself gradually diminished, sliding successively 

 down the hill as it has been undermined, and together with the 

 more gradual abrasions of the land in the upper parts of the valley, 

 hurried by the daily course of the river to the Garry. 



The probability of this supposition is strengthened by the occur- 

 rence of a similar circumstance at no great distance. This may be 

 seen near the junction of the Garry and the Tumel in the hills 

 above Fascally j and the deposit is here also of enormous thickness 

 and of similar materials. Like that in Glen Tilt, it occurs where 

 the valley appears to have favoured the accumulation of a diluvian 

 deposit, and where the subsequent action of the river in its rapid 

 and corroding progress downwards as it runs through the narrow 



2 s 2 



