Hardman — On the Carboniferous Dolomites of Ireland. 727 



mingled with carhonate of lime, has changed them into magne&ite or dolo- 

 mite.''^ I cannot see that this theory differs in any essential respect 

 from that of Von Morlot (see ante, p. 705), Tvhich Hunt himself con- 

 demns ; but it altogether fails to account for what frequently occurs 

 near Kilkenny. 



(1). The dolomites are often interstratified with ordinary blue 

 limestone highly fossiliferous. The section at Riverview is one out of 

 many that shows this. On the evaporation theory we should suppose 

 a most extraordinary series of oscillatbry movements, alternating be- 

 tween a deep and clear sea, fitted to sustain the life of corals and such 

 organisms, and again a land-locked lagoon merging into a salt lake : 

 all this repeated many times, in the process of producing a few hundred 

 feet of the interstratified rocks. This is, I venture to say, inconceiv- 

 able. In order to get a deposit of carbonate of lime alone, at least three- 

 fourths of the sea water must be evaporated, as Bischof has shown ;* but 

 even then the carbonate of magnesia will remain in solution a considera- 

 ble time longer. By the time the water became sufficiently dissipated 

 for the latter to subside, the sea would have become a veritable pickle 

 in which few organic fonns could live.f Yet we have highly fossili- 

 ferous dolomites, which would prove that the animals lived in the sea 

 water during the time of the deposition of those rocks, and that, during 

 a very considerable time besides. In fact, on the evaporation theory 

 we should have only the following distinct groups : — (1), carbonate 

 of lime; (2), magnesite ; (3), gypsum; (4), common salt; but no 

 dolomite. It is quite possible, however, that some dolomites, such as 

 those of the Pennian formation, may have been indirectly the result 

 of evaporation ; thus, that during the process of concentration a 

 gTcater amount of carbonate of magnesia might be assimilated by the 

 animals then living in the lagoon ; and thus that the alteration to dolo- 

 mite might be sooner effected afterwards. It seems to me that it is 

 only by this assimilation, and the subsequent removal of the excess of 

 carbonate of lime, that large masses of dolomite could be formed ; for 

 if we consider the very small percentage of carbonate of magnesia or 

 lime present in sea water, and suppose even a portion of it enclosed in 

 a position favourable to evaporation, it is clear that the beds of sulphate 

 of lime, and of the chlorides, would bear an enormous proportion to those 

 of carbonate of magnesia or lime, or to dolomites. 



(2). Again ; were the dolomites originally deposited chemically, 

 they should form perfectly definite beds — dolomite, and nothing else. 

 It would be perfectly impossible, under any circumstances of evapo- 

 ration, to have the same bed at one place limestone highly fossiHfe- 

 rous, and at another (a few yards off) truly dolomitic, the fossils 



* Chem. Geol., i., p. 177. 



t Except perhaps those remarkable salmon Avhich, as related by Smollett, in 

 " Humphrey Clinker," the Scottish laird kept in a tank, to which he graduallj- added 

 more and more salt, &c. ; so that at last they coiild be taken alive readj^ pickled ! 



