212 PROCEEDINGS OF SOCIETIES. 



of a palc-greenish colour, exceedingly hard, striking fire freely under 

 the hammer ; but, when subjected to long-continued action of the re- 

 ducing flame of the blow-pipe, melting slightly on the edges, particu- 

 larly in the neighbourhood of the minute specks of silicate of iron which 

 appear here and there through the body of the rock. 



The following analysis will serve to give an, exact idea of the com- 

 position of this rock, which is more siliceous than the felstones of Wa- 

 terford and Killarney : — 



Per cent. Atoms. 



Silica, 81-36 . . 1-808 



Alumina, 7'86 0-151) ft 1Q9 



Peroxide of iron, . . . 3-32 0-041 j U ' iyJ 



.Lime, 0-99 0-035 \ 



Magnesia, 0-45 0-022 ( ft on 



Potash, 3-09 0-065 ( U '^ Ut> 



Soda, 2-63 0-084/ 



99-70 



It is evident, from this analysis, that the atoms of protoxides and 

 peroxides are about equal in quantity, and that the rock may be repre- 

 sented by a mixture of felspar and quartz : — 



Q + F = 1*808, 

 P = 0-199. 



Prom these equations we find, that its mineralogical' composition is 

 as follows : — 



Per cent. 



Quartz, 45*54 



Orthoclase felspar, 54-16 



99-70 

 2. Siliceo-felspathic Mocks of Knockmahon, county of Waterford. 



The felspathic rocks of Knockmahon are intimately associated with 

 the copper lodes which have rendered, that locality famous, and occur 

 abundantly on the shore below the village of Bonmahon, in prismatic 

 masses of a columnar structure, which have received the name of the 

 Bishop's Library. These rocks occur also, and are well shown, in a 

 cutting for a tram-road connecting Tankardstown with Knockmahon 

 Mine. In this latter locality they occur stratified conformably with 

 the brown fossiliferous Silurian slates which are found at the Tank- 

 ardstown Mine. The following analysis is of a specimen taken from 

 the stratified siliceo- felspathic rocks of the cutting of the tram-road : — 



