E. Wethered — Organisms in Carboniferous Limestone. 539 



the main joints, and also the intimate connexion which exists between 

 them, supports the conclusion that the change was produced after 

 the operation of those forces which gave rise to the phenomena of 

 joints." Professor Harkness attributes the dolomitization to the sea- 

 water finding its way into the joints and fissures of the limestone 

 when submerged. In the case of the Forest of Dean, however, the 

 amount of jointing would not be sufficient to produce such effects, 

 in the presence of sea- water, as Professor Harkness speaks of in the 

 district of Cork. That sea-water was the agent which supplied 

 the magnesia I do not agree. Professor J. D. Dana states * that 

 " analyses of the Coral Limestone of the elevated coral island Matea, 

 by Professor B. Silliman, jun.,have determined the singular fact that, 

 although the Corals themselves contain very little carbonate of mag- 

 nesia, magnesia is largely present in some specimens of the rock. 

 It affords on analysis, 38-07 per cent, of carbonate of magnesia, and 

 hence only 61-93 of carbonate of lime. . . . This introduction of 

 magnesia into the consolidating submerged coral-sand or mud, has 

 apparently taken place (1) in sea- water at the ordinary temperature ; 

 and (2) without the agency of any mineral waters except the ocean. 

 But the sand or mud may have been that of a contracting and 

 evaporating lagoon, in which the magnesia and other salts of the 

 ocean were in a concentrated state." Looking at the fact that the 

 Carboniferous Limestone in the Forest of Dean was undoubtedly 

 originally formed by accumulations of the remains of organisms, 

 seems to me to be against the lagoon theory. Had the strata been 

 deposited under conditions of concentrating waters, I fail to see 

 how the organisms could have lived in so briny a solution. In 

 endeavouring to arrive at a solution of the problem, there are two 

 details which must be considered. (1) The beds of the Carbon- 

 iferous Limestone are not mixed with argillaceous strata ; those 

 of the Lower and Upper Limestone series are. (2) The proportion 

 of insoluble residue (sand) in the Carboniferous Limestone is small. 

 These two facts seem to indicate that either there was deep 

 water at a distance from land, too far for much sediment to be 

 carried, or else a lagoon existed. My objection to the lagoon 

 theory has already been stated, and I think the evidence is in 

 favour of deep water some distance from land. But in what way 

 would this solve the problem of the formation of dolomite in the 

 Carboniferous Limestone ? We must consider the fact that in the 

 Lower and Upper Limestones there are argillaceous beds ; these 

 latter are less numerous in the former than in the latter, and I am 

 informed by Colchester Wemyss, Esq., J.P., that some of the beds 

 contain a considerable proportion of magnesia. The effect of these 

 argillaceous beds, it seems to me, would be to cover the limestone 

 beneath with a water-tight stratum. This, however, was not so in 

 the Carboniferous Limestone, and consequently the sea-water would 

 have free course through the at first loosely accumulated calcareous 

 remains. That dolomite might, under such circumstances, result 

 from the decomposition of magnesium chloride, and possibly 



1 Corals and Coral Islands, 1872, pp. 356 — 357. 



