Dr. Mac Culloch on the Geology of Glen Tilt, 261 



In examining such a valley as that which is the subject of 

 the present paper, the course of the river, as well as the 

 ranges of hills which bound it, must necessarily enter into the 

 history of its geological formation. Where these are of similar 

 structure it is easy to comprehend the whole in one line of descrip- 

 tion, taking the course of the river for a guide. But in Glen 

 Tilt a dissimilarity so complete is observed between the opposite 

 sides of the valley, and the numerous and interesting phenomena 

 which occur in the bed of the stream itself are so dissimilar to 

 both, that it is impossible to convey an adequate idea of the whole 

 structure without a distinct examination of the three several lines. 

 A perpetual transference of the imaginary spectator from one point 

 to another, would produce as much confusion in the narration, as 

 such a proceeding in examining the ground would obscure the 

 judgment and puzzle the investigation of the real observer. I have 

 •therefore chosea to detail the examination of the three lines se- 

 parately, and have commenced with the river as the most natural 

 if not the most useful method, describing in succession the ranges 

 ^f hills, of which the history is required to render the phenomena 

 which are to be seen in the river intelligible. 



The Tilt has its rise in a small loch not far from that long valley 

 -which is the common division of the waters running east and west 

 to join the Dee on one hand and the Tumel on the other. As it 

 enters that valley it falls immediately into a line tending westward, 

 from which it deviates towards the south-west before its termination 

 in the Garry, the narrow bottom of the glen affording but little 

 room for lateral deviations, and its direction being subject to no 

 material variation. For two or three miles its course lies so much 

 through alluvial matter that it offers nothing to the spectator ; no? 

 does any material feature occur, before its junction with the Tarff. 



