GEOLOGICAL SOCIETY OF DUBLIN. 213 



Per cent. Atoms. 



Silica, 77-20 . . 1-715 



Alumina, 6-54 0-126 \ lqq 



Peroxide of iron, . . . 5-82 0-073 J Uiyy 

 Lime (carbonate), . . . 1*81 



Magnesia, 0-60 0-030 ) 



Potash, 3-69 0-078 ! 0-206 



Soda, 3-03 0-098 J 



Water, 112 



99-81 



This rock, like that at Ballymurtagh, county of Wieklow, is a com- 

 pound of quartz and felspar. If it be regarded as a sedimentary rock, 

 it must be considered as a trappean ash, composed of felspar and fine 

 quartzose mud, with a slight admixture of carbonate of lime. Its mi- 

 neralogical composition is as follows : — 



Per cent. 



Quartz, 40-81 



Orthoclase felspar, 57*19 



Carbonate of lime, 1*81 



99-81 



3. Silieeo-felspathic Traps of Benaunmore, county of Kerry. 



The hornstones, or silieeo-felspathic rocks of Benaunmore, occur in 

 splendid columns, many of which, as described by Mr. Foot, of the Geo- 

 logical survey, are 200 feet in length. The rock is more translucent 

 than the felspathic traps of "Wicklow or "Waterford already described, 

 and presents more of the character of a truly igneous product. Its 

 analysis gave the following results : — 



} 0-277 



Per cent Atoms. 



Silica, 71-52 . . 1-554 



Alumina, 12*24 0-238 



Peroxide of iron, . . . 3-16 0-039 



Lime, 0*84 0*030 



Magnesia, 0-39 0*019 . 



Potash, 5-65 0-120 l 



Soda, 3-36 0-109 



Loss by ignition, . . . 1-20 



98-36 



This rock, like the others already discussed, is a compound of quartz 

 and felspar ; and it is easy to calculate the proportions of the two mine- 

 rals as follows : — 



VOL. IV. 2 F 



