CHEMICAL EXAMINATION OF ROCKS AND MINERALS. 73 



limestone shales have been formed, was one in which animal and vegetable 

 substances abounded ; for they are full of fossils, and the carbonaceous matter 

 is derived from their decay. I may add that many of these fossils are 

 entirely covered by pyrites. 



The formation of bisulphide of iron appears to require an excess of 

 sulphur, and would naturally be most readily formed wherever there was 

 an excess of animal organic matter, — a supposition which is supported by the 

 fact recorded by Mr. Pepys*, and cited by Sir Charles Lyell in his « Manual 

 of Elementary Geology.' It must be admitted, however, that iron pyrites is 

 found in other rocks belonging to the carboniferous formations almost or 

 wholly free from organic matter. The existence of crystals of iron pyrites 

 in granite and trap rocks under circumstances where it would be difficult, 

 if not impossible, for organic matter to intervene, show that that mineral may 

 be formed in various ways. 



It is worth while remarking, however, that crystalline sulphur has been 

 frequently found in mountain limestone, in which there is not much organic 

 matter. 



Had iron been abundant in the neighbourhood, this sulphur would 

 doubtless have been converted into pyrites. 



6. Chloritic Slate and supposed Metamorphic Limestone derived from it. — 

 The ordinary mode of analysing rocks gives no assistance whatever in de- 

 termining the origin of rocks metamorphosed by igneous agency ; it does not 

 even enable us to determine positively whether a rock has been metamor- 

 phosed by heat or not. The remaining experiments which I have to describe 

 were made with rocks which are generally assigned to this class, and which 

 therefore afford examples of the advantages which may be derived from the 

 employment of the method of examination which I have pursued. The 

 rocks which formed the subject of experiment were, a specimen of altered 

 chloritic slate, and two specimens of what is usually considered as meta- 

 morphic limestone. Beds of this class are found in the N.W. of Ireland, 

 sometimes resting on granite, and always associated with such rocks as gneiss, 

 mica-schist, hornblende slate, &c. I think I have established an interesting 

 relationship between one of those beds and a chloritic schistose rock, which, 

 if it be not wholly opposed to the igneous metamorphosis of the calcareous 

 rock, undoubtedly proves that it could not have been subjected to a very 

 high temperature. 



Before describing the calcareous rock alluded to, it is necessary to give an 

 account of the chlorite schist, and the results of my analysis of it. The rock, 

 which contains some crystals of augite or hornblende and magnetic iron, oc- 

 curs in the Townland of Cavan Lower, half a mile east of the town of Stra- 

 norlar in the Co. of Donegal. It effervesces with acids, as most rocks of a 

 similar character do ; and when digested with them, the micaceous part is 

 partially attacked. On being boiled for some time with acids, the chief part 

 of the chloritic and other minerals separate from the quartz. Here an im- 

 portant problem suggested itself, namely, in what state did the quartz exist? 

 Was it formed in the rock by the action of the heat, that is, did the original 

 rock separate under the influence of heat into chlorite and quartz? or was it 

 originally composed of quartz and some other substance, which alone changed 

 into chlorite ? With the view of attempting a solution of this problem, which 

 applies equally to most kinds of schistose rocks, some thin plates of the schist, 

 carefully detached from different parts of the rock, were treated by diluted 

 hydrochloric acid until every thing soluble was dissolved out. The plates 

 were then repeatedly submitted for some time to the successive action of 



* Geological Transactions, vol. i. p. 399, First Series. 



