Dr. Mac Culloch on the Geology of Glen Tilt. 305 



glUaceous slate. Although dark blue or grey Is the predommant 

 colour, it occasionally varies through different shades to pale grey 

 and greenish grey, and Its texture is equally liable to variations. It 

 is a pure carbonat of lime with the exception of the colouring \n-> 

 gredient iron, containing no notable proportion of other earths, 

 except where it is intermixed with the siliceous or argillaceous 

 laminsE. The same is true of the white varieties, except that 

 where they are much mixed with steatite and serpentine they yield 

 magnesia upon being analyzed. 



The dip of the beds is invariably to the south, but the quantity 

 of that angle is not sufficiently constant to render its measurement 

 an object of interest. It seems to vary from five to fifty and even to 

 sixty degrees. In one place and one only I observed a considerable 

 contortion of the beds, and in many others there are fractures and 

 dislocations to be seen. Yet with such partial irregularities we may 

 still safely consider the general parallelism and stratification as regu- 

 lar, and the dip as a medium constant quantity of perhaps twenty 

 degrees. A few porphyry veins are found to traverse these 

 beds, an appearance too common to call for any particular 

 notice. Nor is it possible for want of marks to refer to the places 

 where they occur. Contortions similar to those here described 

 are not unfrequent in Scotland, and they have been often sup- 

 posed to depend on the vicinity of trap or granite. They are 

 however to be seen in many places where neither of these rocks can 

 be found, and I have observed in the island of Sky* a series of stra- 

 tified rocks of which the evenness and parallel horlzontality is per- 

 fect, although they are traversed by trap veins of enormous magni- 

 tude and great frequency. No general conclusions therefore re- 



* See the first paper in this volume. 



Vol. III. 2 q 



