74 REPORT — 1859. 



boiling hydrochloric and sulphuric acid, care being taken, however, to avoid 

 a rapid ebullition, in order not to break or deform them. This treatment was 

 continued until almost every thing soluble in acids was removed. There then 

 remained a residue of beautiful hyaline quartz fragments enveloped in semi- 

 transparent nacreous crystalline-looking scales. These scales being the sili- 

 ceous skeletons left by the foliated chlorite, they dissolved in caustic potash ; 

 so that after a few successive treatments with acids and caustic potash there 

 only remained quartz debris, some of the grains still bearing the impression 

 of the mineral substances which had adhered to them. Before treating the 

 siliceous residue with caustic potash, the nacreous scales and quartz were so 

 intimately mingled, that at first sight it would be difficult to say whether the 

 latter occurred as fragments, or in an unaltered crystalline state. Treatment 

 with caustic potash removes all doubt, however, on this subject. 

 The following are the results of an analysis of the chloritic slate : — 



Alkalies (potash) 0*545 



Magnesia 5*439 



Lime 0965 



Protoxide of iron and a little sesquioxide of iron 1 Q.nfi4< 



derived from the magnetic oxide J 



Alumina 7*360 



Silica and quartz 61*762 



Water 2*862 



Carbonate of lime 11 *08 1 



Carbonate of magnesia 1 *024 



100*102 



The ratio of the oxygen in the protoxide bases existing as silicates in this 

 chloritic slate is to that in the alumina as 4*464' : 3*440, that is, there are 

 four equivalents of protoxide bases to one of alumina, which is exactly that 

 in typical chlorite of the formula 4 (RO, Si0 2 ) A1 2 3 , Si0 2 +3HO. The 

 water in the slate, however, would only correspond to two equivalents; but, 

 on the other hand, the quantity of water in chlorite is subject to vary, and 

 some analyses have been recorded in which the water does not exceed two 

 equivalents. 



The carbonate of lime or magnesia has no doubt been formed by the 

 decomposition of augite or hornblende. From this it would appear that the 

 original material out of which the chloritic slate was formed, consisted of 

 a calcareo-magnesian slate-clay or shale intermixed with hyaline quartz de- 

 bris, that is, a rock resembling in composition rich clay marls. 



Having thus ascertained, with considerable probability of truth, the origin 

 of the chlorite, I shall now endeavour to show that the subsequent decom- 

 position of part of the chloritic rock may have furnished materials for the 

 formation of a rock of a totally different character, which is found in the same 

 parish, and about three miles east of the locality from which this schistose 

 rock was obtained, or about two miles to the west of Castlefinn (both local- 

 ities being north of the river Finn). It is marked as metamorphic limestone 

 on Sir Richard Griffith's Geological Map of Ireland. It has a saccharoidal 

 structure, and a greyish white colour, the grey tint being due to micaceous 

 scales of chlorite which are disseminated through the mass, together with 

 some small crystals of magnetic and common iron pyrites and debris of 

 hyaline quartz. 



A fragment of this rock digested in dilute hydrochloric acid, left a residue 

 analogous to that left by the schist when treated in a similar manner. The 

 residue consisted of debris of fragments of hyaline quartz, sometimes agglo- 



