D. MACKINTOSH ON THE OEIGIN OF SLICKENSIDES. 387 



Mr. Dowker remarked on the production of slickensides in clay 

 from movements due to alternate expansion and contraction by- 

 moisture and drought. He also showed the occurrence of a similar 

 phenomenon in a crystalline mass found in a cavity of the Chalk. 



Mr. Koch remarked that slickensides may be seen in coal, in 

 which case heat could have had no action. 



Prof. Kupert Jones inquired why mere rubbing might not have 

 been the cause as in artificial polishing. 



Prof. Seeley referred to some examples of artificial slickensides 

 caused by striking and rubbing. 



