On the Metawiorphic Rocks of Counties Sligo aTid .Leitrim. 367 



quartzose rocks on each side. There is no merging of one into 

 the other. 



But, on the other hand, how' can we account for the magnetite ? 

 Supposing we admitted the sedimentary character of the rock. I 

 believe there is no case on record of magnetite being found in 

 rocks other than those which have been metamorphosed, or 

 which are of known igneous origin, and in which it is notably of 

 common occurrence. It is also easily produced by the chemist 

 under conditions of intense heat ; but no one has yet succeeded 

 in obtaining it from chemical decomposition in the cold. It may, 

 therefore, be taken for granted that at the time the magnetite 

 was developed in the serpentine that rock must have been sub- 

 jected to a very intense heat — doubtless a moist heat under great 

 pressure. The rock must, therefore, have been altered since its 

 deposition, and therefore could not have been — what it is now — 

 serpentine originally. 



The present case can be easily accounted for by the alteration 

 of ordinary magnesian limestone and shales. This great band of 

 serpentine was perhaps originally a bed of magnesian limestone 

 intercalated between shales and sandstone rocks. Carbonate of 

 lime was gradually eliminated, and replaced by silicate of alumina 

 and magnesium from the accompanying shales, and the oxides of 

 iron under the influence of intense heat and super-heated steam 

 were in part converted into magnetite. 



This is partially confirmed in the present instance by the fact 

 that at the basal junction of the rock with the quartzite the 

 serpentine merges into what has all the appearance of altered 

 carboniferous limestone shale. 



The probability of such a conversion is, of course, beyond all 

 question. Serpentine is of common occurrence amongst altered 

 limestone, as is mentioned by Bischof and others ; and I have 

 myself noticed the actual passage of a magnesian limestone into 

 serpentine near Tramore, county Waterford, where the silurian 

 rocks are greatly invaded by igneous and trap rocks. 



Chrysolite and Tremolite, or Ashestus. — I should mention that 

 the Sligo serpentine contains numerous bands of a fibrous mine- 

 ral which answers to many of the characteristics of chrysolite, 

 but that it contains a very large per-centage of water — as much 

 as 17 per cent., and encloses quantities of magnetite. There are 

 also abundant bands of a mineral which closely resembles tremo- 



