102 



PEOCEEDINGS OF THE GEOLOaiCAL SOCIETY. 



[June 9, 



nexion between the direction of the joints and the courses of the 

 magnesian limestones. In some instances the line of the joint, when 

 this joint is closed, forms the boundary between the dolomite and 

 the ordinary limestone; the two distinct portions terminating 

 abruptly against the sides of the joint. 



All the circumstances in connexion with the mode of occurrence 

 of these magnesian limestones support the conclusion, that in this 

 locality jointing was antecedent to dolomitization, and has given to 

 this form of limestone its peculiar north and south direction. 



The strike of the dolomites is not, in all instances, in this locality, 

 confined to the direction of the joints. In the smaller quarry at 

 Summerstown (the one in which, owing to the thin-bedded and sili- 

 ceous nature of the limestone, jointing is not very apparent) dolo- 

 mite is also met with ; but here, instead of having the north and 

 south course, so common in the district, it is found striking east and 

 west, conforming to stratification. Fig. 11 exhibits this mode of the 



Fig. 11. — Dolomitic Carboniferous Limestone, at Summerstown, 



Co. CorTc. 



The tinted bands are dolomitic. 



occurrence. The strata are vertical, and have a yellowish colour 

 from weathering. The dolomite, however,is brown*. 



Phenomena of a like character, in connexion with jointing and 

 dolomitization, have been well described by Mr. Wyley as common 

 in the county of Kilkenny, among the Carboniferous limestones f. 

 From Mr. Wyley's sections and descriptions, it would appear, that in 

 this country also, there is great reason for inferring, even when dolo- 

 mites conform to stratification, that this substance cannot be regarded 



* I am indebted, in a great measure, to Mr. F. Jennings for having pointed out 

 these localities to me. 



t Journal of the Dublin Geol. Soc. vol. vi. p. 109 et seq. 



