100 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [JuilC 9, 



to observe, whether the occurrence and perfection of jointing are 

 dependent, in other districts, on the amount of disturbance to which 

 the strata have been subjected. 



Mere pressure, however, fails to account for the complex systems 

 of joints which occur in the purer limestones ; and yet it is difficult 

 to conceive that other forces have been ia operation in the production 

 of these complex series. 



It appears probable, from the great purity of the more perfectly 

 jointed limestones, that these, duriag their consolidation, had a dis- 

 position to assume a more crystalline nature than siliceous limestones ; 

 and that, on the application of pressure to these pure limestones, not 

 only they obeyed the influence of the force which gave them, in 

 common with the Devonian rocks and siliceous limestones, a north 

 and south series of joints ; but the pressure caused them to divide, in 

 accordance with the mineral cleavage of calc-spar, and produced in 

 them a disposition to assume a rhombohedral form. Although the 

 case of the rhombohedral jointing of the Silurian shale from Clunnie 

 quarry, before alluded to, seems somewhat antagonistic to this theory, 

 yet the imiform size of the angles of masses intersected by the com- 

 plex systems of joints, their angles agreeing with those of the 

 cleavage-planes of calc-spar, and the pure mineral nature of these 

 limestones, support the opinion that mineral cleavage is in part an 

 important agent in the production of complex series of jointings in 

 limestones. 



On the Dolomites. 



Condition of the Dolomites near Cork. — The connexion between 

 jointing and the occurrence of magnesian limestone will, at first sight, 

 appear very remote. "When, however, we come to examine the divi- 

 sional planes in many loeahties in the district around Cork, it will 

 appear that the presence of joints has had a great influence on the 

 occurrence of dolomites in the Carboniferous strata of this neigh- 

 bourhood. The mode in which the magnesian limestones make their 

 appearance, in the district under review, leads to the inference, that 

 the dolomites were not deposited, by the ordinary action of water, 

 as sedimentary rocks, but that they are superinduced structures, 

 which have not only arisen from the action of forces operating sub- 

 sequent to the deposition and consolidation of the limestones in which 

 they occur, but have had their origin after the operation of that 

 force which has produced joints among these limestone-strata. 



In order to understand the mode of occurrence, and the circum- 

 stances which have given rise to the production of these dolomites, 

 it will be necessary to detail some of the conditions under which 

 they appear. 



In the quarry near the Gasworks, which has been already aUuded 

 to (p. 93), several masses of dolomite are seen ; and these exhibit* 

 such an aspect as to indicate that their mode of occurrence has little 

 connexion with the original bedding of the grey limestone, of which 

 this quarry for the most part consists. On the south side of this 



