82 Scientific Proceedings, Royal Dublin Society. 



scale, provided the proportions are about right. Felspars con- 

 taining much alteration-products, but still preserving their hard- 

 ness, should, so far as the evidence goes, be regarded as liable to 

 wear to a polished surface. 



It is hardly necessary to point out here that, in the same quarry, 

 a rock may vary considerably in character, and for this the 

 engineer must be on the look-out. 



While experience in examining sets will always assist us in 

 our power of making a correct forecast of the behaviour of the 

 set, I trust that the remarks and photographs in this Paper will 

 not be without use to engineers unacquainted with petrography. 

 Many questions have arisen in the course of it which require 

 further investigation. It is my hope that answers to them may 

 be forthcoming. 



