276 Scientiftc Proceedings, Royal DvMin Soeiety, 



rocks, of which, broadly speaking, those in the south-west spur 

 are plutonic, and those to the north volcanic. In many places 

 both the sedimentary rocks and eruptive dykes have undergone 

 a peculiar change or decomposition, being altered into soft friable 

 red and yellow clays, but in which the original structure is still 

 apparent. This is evidently a chemical metamorphbsis, and 

 appears to have been either an impregnation of the rocks with 

 iron salts, or a decomposition of those already present, probably 

 a combination or alternation of both. 



The superficial deposits consist of various depths of meteoric or 

 local drift, the result of the disintegration of the upper portions 

 of the underlying rocks ; over this in places a thin covering of 

 peat, while in the valleys there are old river gravels and alluvial 

 deposits. 



The proved auriferous valleys lie to the north and north-east 

 of the summit of the mountain, the principal and central one at 

 the base of the eastward slope of the northern spur, which com- 

 prises the hills of Moneyteige (1,892 feet) and Ballycoog (1,169 

 feet), in the rocks of which are lodes of magnetite, pyrite, cal- 

 copyrite, and galena. 



The Occurrence of the Auriferous Gravels. 



In the south Wicklow district the valleys that have been 

 proved auriferous have all very similar physical and geological 

 characters ; they are comparatively narrow, with very steep and 

 abrupt sides, on which there is usually only a scanty covering of 

 soil. The bottom of the valleys are comparatively level, formed 

 of alluvial deposits and local drift, that appears to have been 

 derived from the wasting away of the sides of the original chasm 

 in the rock. These general features become modified on ascend- 

 ing the streams. (See Plate xx.. Fig. 1.) 



The gold has been found principally in an ancient river gravel 

 associated with the minerals previously mentioned, which make 

 up the " black sand " resting on the " bed rock " at the bottom 

 of the old valley and under the local drift, in which, and the 

 more recent river deposits, it is distributed but sparingly. The 

 auriferous deposit is richest at the lower portions, especially 



