54 Dr. T. Sterry Hunt — On Chemical Geology. 



chiefly fossilferous, which are found in the earth's crust ? The only plausible 

 chemical explanation is that which I have given, namely — that the chlorid of cal- 

 cium has been decomposed by carbonate of soda derived from decaying feld spathic 

 rocks, giving rise thereby to common salt and to the carbonate of lime which has 

 supplied the marine animals. 



As regards the question on the origin of dolomites, which Mr. Forbes next pro- 

 ceeds to notice, he will do well to consult my paper on the subject in the American 

 Journal of Science for July, 1866 ( [2] xlii. 49). In this, at §112, he will see that, 

 apart from the formation of stratified sedimentary dolomites, I insist upon the fre- 

 quent occurrence of dolomite as a mineral of secondary deposition, lining drusy 

 cavities, filling veins, and even the moulds of fossil shells. To such cases the 

 observations of Sorby may possibly refer. I can find no other account of his re- 

 searches than the brief note in the Proc. of the Brit. Assoc, for 1856, cited by Mr. 

 Forbes. Although I have a great respect for Mr. Sorby as an investigator, I have 

 very little for the old theoiy of dolomitization of sedimentary limestones. No one 

 who has carefully studied, as I have done for years, the distribution and association 

 of the great beds of dolomite which occur in the Lower Silurian rocks of Canada 

 and New England, can for a moment admit that these are the products of subse- 

 quent alteration. Repeated alternation of pure blue limestones with reddish 

 ferruginous dolomites, interrupted beds and patches of these enclosed in the 

 former, the line of demarcation sharply drawn, and finally conglomerates in which 

 pebbles of pure limestone are enclosed in beds of dolomite, are incontrovertible 

 evidences against the theory of the dolomitization of limestones, and in favour of the 

 deposition of dolomites as magnesian sediments. (Geol. of Canada, 1863, p. 612). 

 Mr. Forbes, in a note, insinuates that I am unaware of the various speculations 

 and theories which have been put forward to explain the supposed origm of dolo- 

 mite by alteration. Although the stratigraphical relations of dolomite, as described 

 above, completely contradict this hypothesis of its origin, at least in the great 

 majority of cases, Mr. Forbes will find that the observations and speculations of 

 Haidinger, Von Morlot, Marignac, and others, on this subject have been fully dis- 

 cussed and made the subject of multiplied experiments by me in a memoir published 

 in 1859 {Amer. Journ. Science, [2] xxviii. 170, 365), and later in the paper quoted 

 above, and that I have shown by many experiments that the action of sulphate of 

 magnesia on carbonate of lime, alluded to by Haidinger and Von Morlot before 

 Harkness or Regnault, does not give rise to dolomite, but to carbonate of magnesia, 

 which remains mechanically intermingled with sulphate of lime and any excess of 

 carbonate of lime. 



Mr. Forbes says that some of the results of my prolonged study of certain of the 

 salts of lime and magnesia, which are, for the most part, set forth in the papers 

 just referred to, were considered by me to be worthy of being presented to the French 

 Academy {Comptes Rendus, April 22, 1867) although he declares the reactions 

 therein described, to have been for more than twenty years in general application, 

 on a large scale in Great Britain for the manufacture of magnesia salts. Here it 

 becomes difficult to admit the plea of ignorance which suggests itself for most of 

 Mr. Forbes previous errors and mis-statements. I have, in the note to the French 

 Academy, above referred to, pointed out the following facts, discovered by my in- 

 vestigations of the salts of hme and magnesia : — 1st. That bi-carbonate of lime, at 

 ordinary temperatures, decomposes solutions of sulphate of soda and sulphate of 

 magnesia, with formation of sulphate of lime and bi-carbonates. 2nd. That from 

 mingled solutions of sulphate of magnesia and bi-carbonate of lime, there separates 

 by evaporation, crystalline gypsum, and, subsequently, a hydrous carbonate of 

 magnesia ; the bi-carbonate of this base being, as is well known, very much more 

 soluble than either the sulphate or the bi-carbonate of lime. 3rd. That this sepa- 

 ration of gypsum is favoured and rendered more complete by an atmosphere 

 impregnated with carbonic acid gas ; and 4th, that mixtures, in due proportions, 

 of precipitated carbonate of lime and hydrous carbonate of magnesia, when gently 

 heated under pressure, and in the presence of water, unite to form the anhydrous 

 double carbonate, dolomite. These are the reactions which I described to the 

 French Academy as new, and as forming the basis of a reasonable theory of the 

 origin of gypsums and of dolomites. I now demand Mr. Forbes to make good 

 his bold assertions to the contrary, or to show that any one of them has been 

 employed for the last twenty years in the manufacture of magnesian salts. 



