94 ROCK-METAMORPHISM. 



well-developed joints of the rock 1 *; for it is a fact that they 

 (coralloidal forms) are present on the grandest scale in immediate 

 connexion with these divisional structures, shooting off from 

 their planes, and appearing as if they were globose nullipores, 

 stromatoporal sponges, branching corals, and other organisms f. 



This view of the demagnesiation of the coralloidal limestone 

 seems to be perfectly warranted, since it is paralleled in the known 

 cases of calcite occurring as a pseudomorph after miemite. 



According to our hypothesis of the dolomitization of the 

 north-of-England Permian rocks, the magnesiated water has 

 descended from overlying or subaerial sea-basins. But possibly 

 in other cases, as in Ireland and the north of England, where local 

 examples are common of ordinary Carboniferous limestone sud- 

 denly becoming changed into dolomite, the water may have as- 

 cended, through jointings and deposition-partings, from subter- 

 ranean sources. We entertain no doubt, however, that the 

 water was originally derived from the sea. 



There remains to be noticed another methylotic hypothesis, 

 which has been suggested by ApjohnJ and Grandjean. Mr. 

 Hardman adopts it in explanation of the Irish local examples 

 referred to in the last paragraph . He assumes that the lime- 

 stone in these cases originally contained carbonate of magnesia, 

 derived from corals and other organisms, a few per cent, (rarely 

 7) having been found in some living species. In limestones of 

 the kind, when penetrated by water charged with carbonic 

 acid, this solvent, it is suggested, would act on their calcareous 



* There are beds containing fossils at " Byers's Quarry," on the coast a 

 few miles north of Sunderland, which have been demagnesiated, but without 

 development of coralloidal structures ; they, however, are highly crystalline. 



t At the time of writing my Monograph of the Permian Fossils (see 

 Introduction, pp. xiv-xxi) I was of opinion that the Permian limestones of 

 Durham were originally dolomitic, but that in certain localities, as near 

 Sunderland, they had undergone methylosis, the remarkable configurations 

 being the result. A fine collection of these configurations, formed by myself 

 during a period of several years, is contained in the Geological Museum of 

 the Queen's College, Galway. An inspection of them will fully bear out the 

 description of their similarity to certain organisms. W. K. 



f " Analysis of some Irish Dolomites," Journal of the Geological Society 

 of Dublin, vol. i. pp. 368-381 (1838). Apjohn, however, is more in favour of 

 the rocks he noticed having been originally dolomitic* 



Proc, R, Irish Academy, no. 7, pp. 705-730 (1876). 



