IN GLEN TILT. 



311 



particularly the case between the Crochie, and a point more 

 than three quarters of a mile below Forest Lodge, and 

 opposite to M in the map, and the plan of the river. 



19. Such a difference of aspect in the surface, gave reason 

 to expect a difference in the nature of the rock beneath. Ac- 

 cordingly, on the south or south-east side, we found it to con- 

 sist of strata, of which the chief materials were granular lime- 

 stone, granular quartz, gneiss, and mica-slate. The general 

 stretch of these strata coincides nearly with the direction of 

 the valley, and their dip is towards the south-east, into the face 

 of the mountain, at a large angle ; though the stretch and dip 

 are occasionally somewhat varied by inflexions. On the north 

 or north-west side, the mountain is chiefly composed of grey 

 sienite, or rather sienitic greenstone, which bears no marks of 

 stratification. In the bottom of the Glen, therefore, is the line 

 of junction between the strata on the south-eastern side, and 

 the unstratified masses on the north-western ; and it seems as 

 if the course of the Tilt had here been very much determined 

 by the position of the limestone strata, which occur frequently 

 throughout the Glen, on the left bank of the river, and have 

 been cut through by the stream. The facility with which 

 limestone is worn away by water, in comparison with most of 

 the other materials of a primary country, must have struck 

 every mineralogist, who has met with it in that situation. 



20. This general account of the rocks, that form the 

 opposite sides of the valley, rests on the following observa- 

 tions. 



21. We saw no sienite on the south side of the river, except 

 immediately on its bank. The rock, where it appears, within a 

 hundred feet above it, is stratified, and often of limestone. In 

 order to ascertain its nature higher up, we ascended the moun- 

 tain in two places, following, in each, a line almost at righ; 



Rr 2 angles 



