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ON THE PETEOLOGICAL EXAMINATION OF ROAD-METAL. 



By J. JOLY, D.Sc, F.E.S., 

 Professor of Geology and Mineralogy in the University of Dublin. 



[Plate XXXII.] 

 [Read, March 21 ; received for Publication, March 24 ; published, May 13, 1905.] 



In a Paper on the application of petrologieal methods to the 

 examination of paving-sets (these Proceedings, vol. x., N.S., 

 p. 63, et seq.), I arrived at results sufficiently promising to suggest 

 that microscopical study might also prove of use in the selection 

 of road-metal. I postponed the question until I should have 

 acquired some practical data hearing on material in use. In 

 November, 1903, Mr. T. Aitken, m.inst.c.e., of Cupar-Fife, 1 was so 

 good as to send me a collection of sixteen various metals in use on 

 the roads in his district. Along with these he forwarded informa- 

 tion as to his experience of their qualities. Pressure of other work 

 has hindered me from publishing the petrologieal notes I then 

 immediately made upon the microscopic character of these rocks. 



General Considerations. 



A road-metal may fail from (a) being deficient in hardness 

 and coherence, (b) feeble weathering qualities, and (c) being 

 deficient in cementitious or binding quality. 



Failure under {a) results in a rapid development of mud in 

 wet weather, or of dust and sand in fine weather — in short, what 

 may be described as bad wearing properties. A hard stone is 

 not necessarily a good wearing stone. Something more than 

 hardness is required. Thus a hard rock may possess cleavage in 



1 Author of " fioad-Making and Maintenance" : a Practical Treatise for Engineers, 

 Surveyors, and others, 1900. 



