The Magnesian Limestone of the Cork District. 



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worked for the "Magnesia-stone" only, the space left by the 

 stone removed consequently representing the original dolo- 

 mitic deposit. 



4. The upper surface of both the limestone and dolomite is 

 found to be eroded by the action of water or ice, and is often 

 covered, by deposits containing sand, gravel, or rounded 

 stones. I have obtained a sample of gravel from over a long 

 band which was found to the westward of the fault, but not in 

 line with it {^fig. i, ^) ; it has not 5'et been ascertained whether 

 this is a continuation of the aforementioned strip {a) or no ; at 

 present it seems more in character with another {c) which 

 runs parallel a short distance to the south. 



5. The line of demarcation between the magnesia and lime- 

 stone when found in juxta-position is always sharp; even 

 when the minerals are nearh' of the same colour and general 

 appearance, it is plain to a practised eye : and while the}* often 

 adhere, so that a hand-specimen containing both may be 

 obtained, the}*, on the other hand, can in general be readily 

 separated at the surface of junction. 



The following anah'ses of samples taken from the same 

 stone, not a 3'ard apart, will illustrate the great difference 

 of composition between rocks which an ordinary' obser\'er 

 would take to be identical: — 



6. The dolomite is generall}' less pure than the limestone. 

 This point is also shown b}' the above anah'sis (although the 

 sample of dolomite is an exceptionally pure one), there being 

 nearh' fifty per cent, more foreign matter present in the latter 

 than in the sample of limestone. In man}' cases the amount 

 of foreign matter is much greater, and often there is so much 

 iron present as to produce a ver\' dark-coloured stone. 



7. In the ca\'ity left b\' the removal of the band of dolomite 

 last mentioned {^fig. i, r), there appear adhering to the lime- 

 stone on each side, slatj' scales — some light-coloured and some 

 of darker colour, and greasy to the touch, — which will pro- 

 bably repay a careful examination. Apart, however, from 

 their composition, they form a strong link in the chain of 

 evidence which goes to prove that the magnesia-stone was 



