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



But however defective and however misplaced in point of dis- 

 tance many of the most remarkable boundaries can scarcely fail of 

 being, it is sufficiently accurate in the general features for all geo- 

 logical purposes. The sections are necessarily general, and are 

 given not as actual sections of any particular points, but as illus- 

 trative of the relative superpositions of the rocks. 



The course of the Tilt may therefore be considered as bounded 

 on one side by the outer edge of the granite mass of the Gram- 

 pians, and on the other by the primary rocks which follow and 

 are superimposed on it. These rocks consist of an alternation of 

 limestone, schist, and quartz rock. 



The bed of the river is cut upon the line of contact of these two 

 separate classes of rock, lying upon the surface of the granite, and 

 against the elevated edges of the stratified rocks. 



Its action has in various instances exposed the junction of the 

 granite with the stratified rocks, and these exposed parts are the 

 confused mixtures in the bed of the river which have already 

 been described 



Although the river follows this line of junction in a general 

 view, it does not follow it so accurately as always to keep the 

 granite on its right bank, and the stratified rocks on its left. 



For this reason the stratified rocks are sometimes seen crossing to 

 the right bank, and even ascending high up the right side of the 

 hill. The granite also crosses to the left in a few cases, but as it 

 dips under the stratified rocks it is not found in the hill. 



The Sections which are given illustrate this variation and explain 

 its cause. 



The apparent alternation of the granite with the stratified rocks 

 is also explained by attending to this arrangement. 



The granite masses which extend beyond the general surface of 



