IN GLEN TILT. 319 



grains ; and so likewise does the quartz *. Minute crystals of 

 iron pyrites are sometimes interspersed ; and our specimens ef- 

 fervesce slightly with muriatic acid. At a point marked C in 

 the plan of the river, there is a variety of sienite, or rather sie- 

 nitic greenstone, which I shall have occasion to describe here- 

 after. 



38. In the bed of the river there are four spots, that draw 

 peculiar attention by affording a good exhibition of the rock, 

 and three of them contain the most distinct phenomena of the 

 junctions of the strata with the sienite. They are marked in 

 the plan of the river by the letters A, B, C, and D, and I shall 

 employ these as names in describing them. 



39. The spot marked A is the first place where the sie- 

 nite is met with, in ascending the course of the Tilt. It 

 may be readily found by a singularity in the channel of the 

 river. Some large irregular masses of rock project on either 

 side from the banks, and through them the water has cut a 

 deep and narrow chasm. Above these rocks, the river holds 

 a course nearly coinciding with that of the valley; on meeting 

 with them, it makes a sharp turn to the southward, and, after 

 falling a few feet, in its passage through the chasm, expands 

 into a pool at the outlet, and resumes its former direction. 

 The direction of the channel through the chasm is near- 

 ly N. and S. 



S s 2 40. At 



* This red sienite bears a close resemblance to that of the Malvern Hills in 

 Worcestershire. I do not remember to have seen one like it any where else. 



