occurring near Knockmahon in the County ofWaterford. 15 



Mean Results. — Taking the means of the two analyses made 

 in duplicate, and tabulating the results for facility of comparison, 

 we obtain the following : — 





Elvanite. 



Felsite. 



Columnar 

 slate. 



Metamorphosed 

 slate (?) 



Water 



1-83 

 7233 

 902 

 6-34 

 106 



i-92 



trace 

 1-46 

 5-83 



1-38 

 8050 

 8-33 

 3-44 

 •96 

 trace 

 121 

 trace 

 1-89 

 212 



•80 



74-28 



778 



710 



-90 



1-08 



•46 



205 



5 05 



5-50 

 52-66 

 18-33 

 10-43 



900 



1-76 



trace 



•73 



212 









Protoxide of iron 



Oxide of manganese ... 







Soda 







9979 



9983 



99-50 



10053 



Loss in water-bath . . . 



•35 



•37 



•34 



•50 



It will be observed that the two specimens of rock most un- 

 like in general appearance, and which, when microscopically 

 examined, were found to differ most essentially in their struc- 

 tural arrangement, resemble each other very closely in point of 

 chemical composition. In fact the ultimate chemical constitu- 

 tions of the elvanite and of the columnar clay-slate are so nearly 

 alike, that a slight change in the latter, resulting in the rear- 

 rangement and crystallization of certain of its constituents, 

 might evidently lead to the production of the former. 



The compositions of the two other rocks differ essentially, not 

 only between themselves, but also very materially from those of 

 the specimens to which we have referred above. 



Analyses. — In making these analyses, the following general 

 routine was adopted. 



The amount of moisture present was ascertained by heating 

 a weighed quantity of the finely pulverized rock in a water-bath 

 until it ceased to lose weight, when it was strongly ignited and 

 the further loss noted. 



About thirty grains of the powdered substance was intimately 

 mixed with three times its weight of carbonate of soda in a pla- 

 tinum crucible. This was enclosed in a clean earthen one, and 

 fused in an ordinary air-furnace supplied with hard coke. 



The fused mixture, after being allowed to cool, was treated 

 with hydrochloric acid and evaporated to dryness. The dried 

 residuum was then moistened with hydrochloric acid, water 

 added, and solution of the chlorides aided by ebullition. The 

 silica, now separated by filtration, was washed, dried, and ignited. 

 From the filtrate, after peroxidation of the iron, oxide of iron and 



