PROCEEDINGS OF GEOLOGTCAL SOCIETIES. 



305 



creation and of occasional migrations, — the conditions of contemporaneity, — the 

 relations of geographic and epochal centres of life, — the laws of recm-rence of 

 animal forms, or their reappearance in new epochs, and of the disappearance of 

 fauna^, — and lastly the points of similarity and of dissimilarity between the 

 paljBOzoic rocks of Wales and Xew York were, in the concluding portion of the 

 memoir, fully dwelt upon. 



June 9, 1858. — 1. " On Jointings, and on the Dolomites near Cork." By Pro- 

 fessor Harkness, F.E.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 occm% cutting through the strata at nearly right angles to 

 their strike. The carboniferous rocks are also intersected by jomts 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 jointing is confined to such as prevail in the Devonian strata, 

 these ha%'ing only the north and south system of divisional i^lanes. Respecting 

 the north and south series of joints, tlie 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 riylit an-les to the direction of the principal joints. 



In connexion with these north ami south joints, both in the limestones of the 

 carboniferous series and also in the underlying carboniferous slates, the author has 

 noticed distortions 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 ali-eady noticed the elongation of the fossils in the dii'ection 

 of the strike of cleavage, a mode of elongation at variance vnth that described by 

 Prof. PhiUips 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 influence in producing Joints. Concerning the other two 

 systems of jomts, which are peculiar to the purer limestone, these have a disposition 

 to split the masses of limestone into rhomboids ; and these di\'isional planes, the 

 author considers, result fi'om the same cause as the principal joints; but, omng to 

 the pure state of the limestone, 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. They 

 usually are met with in a dyke-like form, and in general are fomid conforming to 

 the strike of the perpendicular north and south joints, or, in other words, intersect- 

 ing the strata at right angles. 



In some instances, however, they have an east and vrest course, and are found 

 associated, in this condition, with the siliceous limestones which possess only the 

 principal system of joints. 



These dolomites, from their mode of occm'rence, have had their origin subsequent 

 to the production of the joints, and are often found thinner below than above. 

 From the circumstances of their occurrence, the author is disposed to regard them 

 as metamorphosed ordinary limestones, and the metamorphic action he is disposed 

 to attribute to sea-water, which, containing sulphate of magnesia, has found access, 

 by means of the j oints, into the limestone masses ; and this at an elevated temperature, 

 combined with pressure, has produced these changes, by the double decomposition 

 of sulphate of magnesia and carbonate of lime, and, by this means, given rise to 

 the production of the dolomites occm-ring in this neighbourhood. 



