Prof. Harkness on Jointings, and on the Dolomites near Cork. 385 



June 9. — L. Horner, Esq., Vice-President, in the Chair. 



The following communications were read : — 



1 . " On Jointings, and on the Dolomites near Cork." By Professor 

 Harkness, F.R.S., F.G.S. 



The strata in the neighbourhood of Cork consist of Devonian 

 rocks, and the lower portion of the Carboniferous series. In the 

 former a regular system of north and south joints occur, cutting 

 through the strata at nearly right angles to their strike. The car- 

 boniferous rocks are also intersected by joints having a similar 

 direction ; but in these latter rocks other joints occur, one system 

 of which approaches to the horizontal, and the other is inclined 

 sometimes towards the east, at other times towards the west. This 

 threefold system of jointings, which make their appearance in the 

 carboniferous rocks, is not equally prevalent in all the members of 

 this series. 



The limestones in which they occur in the greatest perfection are 

 such as have the greatest amount of carbonate of lime, and are of 

 the purest character. 



In some siliceous limestones, which are associated with the more 

 perfectly jointed rocks, the series of jointings is confined to such as 

 prevail in the Devonian strata, these having only the north and 

 south system of divisional planes. Respecting the north and south 

 series of joints, the author is disposed to regard them as resulting 

 from pressure, caused by the movements which have given to the 

 strata in the south of Ireland an arrangement in the form of rolls, 

 having an east and west strike, — a course at right angles to the 

 direction of the principal joints. 



In connexion with these north and south joints, both in the lime- 

 stones of the carboniferous series and also in the underlying carbo- 

 niferous slates, the author has noticed distortion of fossils, and this 

 distortion consists of an elongation between the series of joints, the 

 fossils being pulled as it were out towards the sides of these planes, 

 or at right angles to the strike of the joints. 



Prof. Haughton has already noticed the elongation of the fossils 

 in the direction of the strike of cleavage, a mode of elongation at 

 variance with that described by Prof. Phillips and the late Mr. D. 

 Sharpe. 



As this direction (of distortion), however, occurs among strata in 

 the north of Ireland devoid of cleavage, the author is disposed to 

 regard it as resulting rather from jointing than from cleavage ; and 

 he regards it as evidence that pressure has exerted considerable in- 

 fluence in producing- joints. Concerning the other two systems of 

 joints, which are peculiar to the purer limestone, these have a dis- 

 position to split the masses of limestone into rhomboids ; and these 

 divisional planes, the author considers, result from the same cause 

 as the principal joints ; but, owing to the pure state of the lime- 

 stone, lines of division akin to mineral cleavage have intersected 

 these strata, and given to them their complex series of jointings. 



Dolomites also occur in the limestones of the district around 

 Cork. Tliey usually arc met with in a dyke-like form, and in 

 general arc found conforming to the strike of the perpendicular 



