THE PERMIAN FORMATION. 20;i 



decomposition of solutions of the sulphates or chlorides of lime and 

 magnesia by waters of rivers or springs containing bicarbonate of 

 soda, (a) [b) 



Dr. Sorby, on the other hand, holds to the belief that the Magnesian 

 Limestone was at first in great part organic, but that the rock has 

 since become so thoroughly crystalline as to have almost entirely lost 

 its original structure — that some chemical replacement has taken 

 place, though there may have been a certain amount of chemical pre- 

 cipitation at first— and that a combined organic and chemical origin 

 appears more probable than either alone, though it may be difficult to 

 decide to what extent each was instrumental in producing the main 

 mass of the deposit on account of the original structure having been 

 so completely lost, (cj 



My own studies among the Magnesian Limestone deposits of the 

 north-east of England, so far as they have gone, have certainly led me 

 to accept the theory of the direct chemical origin of the bulk of these 

 rocks as the correct one. The crystallisation of the magnesian salts 

 was, I believe, synchronous or nearly so with the formation of these 

 dolomites and the entombment of their organic remains. The general 

 paucity and the stunted aspect (dj of the marine fauna seem to 

 demand some such special explanation, as the co-existence of concen- 

 trated mineral waters suggested by Eamsay. When fossils are present 

 in these rocks, they or their casts are clearly enough defined at certain 

 horizons, even when the rock is most coarsely crystalline, while the 

 intervening beds are to all appearances perfectly destitute of organic 

 remains. The most minute forms of life — foraminifera and entomo- 

 straca — are found at times, though in small numbers, yet perfectly 

 well preserved. How can the bulk of the rock have been recrystallised, 

 and the larger fossils or shelly debris wholly obliterated without at the 

 same time all trace of these minute organisms being destroyed ? Truly 

 certain beds of Magnesian Limestone (e.g., shelly and .polyzoan lime- 

 stones in Durham and Yorkshire) are almost entirely composed of the 



(a) Chemical and Geological Essays, by T. Sterry Hunt, LL.D., reprinted 

 I.sT.'j. p. 90. 



tin It is to be noticed that while both these reactions give us the elements of 

 dolomite, neither of them produces directly dolomite itself. The union of the 

 carbonates into true dolomites or dolomitic Limestones. Dr. Hunt thinks, must 

 have been brought about afterwards by the aid of pressure and elevated tem- 

 perature. Bearing in mind, however, the tendency towards the formation of 

 double salts which magnesia is stated to exhibit, it seems not impossible that 

 combination may take place slowly by simple chemical affinity. We cannot 

 say then that the problem of forming dolomite by direct precipitation has yet 

 been solved experimentally. Imperfect, however, as has been at present the 

 success of experimenters, they have got quite far enough to justify the bi 

 that the process consisted of some reaction betwee n calcareous and mafmesiau 

 salts in solution. What those salts were and what was the exact nature of the 

 reaction have vet to be learned.— " Geologv for Students," by A. H. Green, M.A., 

 P.G.S., 2nd ed., p. 206. 



(c) Anniversarv Address to Geological Soeietv, H. C. Sorby, LL.D., P.R.S., 

 1879, Q.J.G.S., vol. xxxv,, part 2, p. 39. 



(d) All the forms of mollusca found in the Magnesian Limestone of the 

 north-east of England, with scarcely a single exception, are small and dwarfed 

 in aspect compared with their carboniferous congeners, when such there are,— 

 Ramsay, A.C. (Loc. cit.) 



