14 



MINERALOGY OF THE OCHILS. 



bella yeliow. It sometimes, though rarely, is amyg-^ 

 daloidal : the cavities of such varieties, are filled 

 with green earth, and white calcareous spar : crys- 

 tallised felspar is dispersed throughout the mass. 



The claystone alternates with the grey sandstone 

 and the tuff, between which it is most probably 

 the connecting link ; for it passes almost imper- 

 ceptibly at its extremities into each of them. It 

 is doubtful, whether or not the claystone be of 

 older formation than the limestone, as I have seen 

 both in similar relations to the sandstone. The 

 present location, therefore, of these rocks, in so far 

 as they regard each other, must, until more ex- 

 tended observations shall have been made, be con- 

 sidered as entirely arbitrary. 



VII. Tuff. 



A very remarkable tufacious rock occurs above 

 the claystone at the base of Birkhill, and at the 

 western extremity of Wormit Bay. In both of 

 these situations, it alternates with the sandstone 

 and claystone. In no other part of the Ochils 

 have I observed a similar arrangement. 



This tuff is coarse, inclosing portions of felspar, 

 which is sometimes lost in the prevailing mass. 

 The chief colours are, flesh- red and Isabella yel- 

 low. It appears to be one of the newest mem- 

 bers of the sandstone series, and there is a gradual 

 passage from it to the claystone. It is distinctly 



