422 THE CRYSTALLINE SCHISTS OF THE HIGHLANDS. 



relied upon in determining the original succession of the rocks. Notwith- 

 standing this, and the fact that the foliation-planes are themselves often so 

 intensely folded as to make it difficult to determine even the apparent 

 succession of these secondary planes, yet the relative positions of the 

 principal zones given in this paper seem to be fairly well made out. 



Where the foliation-planes have themselves been folded it is interesting to 

 note that the axes of the folds generally head N.W., the overfolding facing 

 S.E., the upper limbs being generally the longer limbs, and the under 

 ones those which are most thinned. As regards the age of these rocks it 

 seems impossible at present to give any definite answer. That they are 

 older than the Lower Old Red Sandstone is evident, from the fact that 

 pebbles of the schists are embedded in the conglomerates of that formation 

 along the Highland frontier. 



Along the Highland frontier and immediately N. of the great line of 

 fault the officers of the Survey have discovered a narrow belt of carbonaceous 

 shales, schists and cherts, the latter containing forms resembling Radiolarian 

 tests, which they consider may be of Arenig age. These appear to have been 

 wedged in between the Highland schists and the Old Red Sandstones, and 

 can be traced at intervals through Perthshire and Dumbartonshire into the 

 Island of Arran. 



The views expressed in this paper are here offered only as a tentative 

 solution of a difficult problem, upon which it is as yet impossible to speak 

 with assurance. 



