1877.] Prof. Hughes, On the base of the Camhriaii Rocks. 89 



the shock felt when the one or the other resistance in addition 

 to the resistance of his body was placed in the path of the 

 discharge. His results therefore are derived from the comparison 

 of the sensations produced by an enormous number of shocks 

 passed through his own body. 



The skill which he thus acquired in the discrimination of 

 shocks was so great that he is probably accurate even when he 

 tells us that the shock when taken through a long thin copper 

 wire wound on a large reel was sensibly greater than when taken 

 direct. The experiment is certainly w^orth repeating, to determine 

 whether the intensification of the physiological effect on account 

 of the oscillatory character of the discharge through a coil would 

 in any case compensate for the weakening effect of the resistance 

 of the coil. But I have not hitherto succeeded in obtaining this 

 result. Indeed on comparing the shock through two coils of equal 

 resistance, one of which had far more self-induction than the other, 

 I found the shock sensibly feebler through the coil of large self- 

 induction. 



Kovemher 19, 1877. 

 Prof. Liveing, President, in the chair, 



A communication was made to the Society by 



Prof. Hughes, On the base of the Cambrian Rocks in North 

 Wales. 



Prof. Hughes read a paper in which he described the lower 

 beds of the Cambrian Rocks near Bangor and Carnarvon. Among 

 these he recognised the Arenig Beds, Lingula Flags, and Harlech 

 Grits, in the lowest part of which were purple and green slates 

 and, at the base, invariably a coarse grit and conglomerate. 



Below the recognised Cambrian Bocks came a series of slates, 

 breccias and porphyries, and these, where they could be followed, 

 were found to rest upon quartz felsites which seemed to pass 

 down into a coarse crystalline quartz-felspar rock which had 

 variously been called Syenite, Syenitic porphyry, or felspar 

 porphyry ; a formation which as it stands looks like rocks 

 usually known as igneous but which he thought might well be 

 of metamorphic origin. 



He had not found evidence of any discordancy betwen the 

 older granitoid rocks of Carnarvon and the 'overlying quartz 

 felsites, or between them and the green slates, hornstones, breccias, 

 and porphyries of Bangor. 



Vol. III. Pt. hi. 7 



