104 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 9, 



sian salts in solution in sea-water, as dolomitizing agents, has been 

 noticed by Mr. Sorby*. 



In the case of the magnesian limestones taking the direction of 

 the joints, we have evidence of sea-water finding its way into these 

 fissures, and penetrating the limestone-masses from these joints ; 

 and in the case where the dolomites conform to the stratification, as 

 in the case of the thin-bedded siliceous limestones which are only 

 partially jointed, the sea- water appears to have had access into the 

 lines of stratification, and to have given to the strata, to some extent, 

 a dolomitic character ; such as, at first sight, would appear to have 

 resulted from a combination of carbonate of lime and carbonate of 

 magnesia held in solution by water, and gradually deposited there- 

 from in the usual manner, under which calcareous rocks have com- 

 monly originated. 



In connexion with the inference which attributes dolomites to the 

 decomposition of magnesian salts in sea-water, it is important to 

 observe, that, even in cases where we have regularly-bedded dolo- 

 mites, these have very frequently associated with them gypseous 

 strata ; a circumstance, which leads us to conclude that, even in the 

 case of stratified magnesian limestones, these owe their dolomitic 

 nature to the same decompositions which produce dolomites from 

 consolidated ordinary limestones. In the stratified dolomites we 

 have, however, the product of the decomposition, gypsum, intimately 

 connected with the other product, magnesian limestone ; a circum- 

 stance, which could not take place under such conditions as give rise 

 to dolomitization in such calcareous rocks as have been consolidated 

 and fissured in the form of joints antecedently to the operation of 

 that force which produced magnesian limestones from ordinary cal- 

 careous rocks. 



P.S. — Since the foregoing communication was read before the 

 Society, the memoir of the Rev. Prof. S. Haughton, "On the 

 Physical Structure of the Old Eed Sandstone of the County of 

 Waterford, considered with relation to Cleavage, Joint-surfaces, and 

 Paults," communicated to the Royal Society, has been published. 

 In the portion of this memoir which has reference to Joints, Prof. 

 Haughton has also arrived at the conclusion that these structures 

 were produced subsequently to the consolidation of the rocky masses 

 which they intersect ; and also after the cleavage -planes had mani- 

 fested themselves in the strata of the district (Phil. Trans, for 1858, 

 p. 338).— E. H., January 1859. 



* Eeport of Brit. Assoc. 1856, Trans, of Sections, p. 77. 



