506 ADDRESS OF T. STERRY HUNT. 
As regards the hypotheses put forward to explain the supposed 
dolomitization of previously-formed limestones by an epigenic. 
process, I may remark that I repeated very many times, under 
varying conditions, the often cited experiment of Von Morlot, who 
claimed to have generated dolomite by the action of sulphate of 
magnesia on carbonate of lime, in the presence of water at a some- 
what elevated temperature under pressure. I showed that what 
he regarded as dolomite was not such, but an admixture of carbon- 
ate of lime with anhydrous and sparingly soluble carbonate of 
magnesia ; the conditions in which the carbonate of magnesia is 
_ liberated in this reaction, not being favorable to its union with the 
carbonate of lime to form the double salt which constitutes dolo- 
mite. The experiment of Marignac, who thought to form dolomite 
hy substituting a solution of chlorid of magnesium for the sul- 
phate, I found to yield similar results, the greater part of the mag- 
nesian carbonate produced passing at once into the insoluble 
condition, without combining with the excess of carbonate of lime 
present. The process for the production of the double carbonate 
described by Ch. Deville, namely, the action of vapors of anhy- 
drous magnesian chlorid on heated carbonate of lime, in accord- 
ance with Von Buch’s strange theory of dolomitization, I have not 
thought necessary to submit to the test of experiment, since the 
conditions required are scarcely conceivable in nature. Multiplied 
geognostical observations show that the notion of the epigeni¢ 
production of dolomite from limestone is untenable, although its 
resolution and deposition in veins, cavities, or pores in other rocks 
is a phenomenon of frequent occurrence. 
The dolomites or magnesian limestones may be conveniently 
considered in two classes ; first, those which are found with gyp- 
sums at various geological horizons; and second, the more abun- 
dant and widely distributed rocks of the same kind, which are not 
associated with deposits of gypsum. The production of the first 
class is dependent upon the decomposition of sulphate of magne- 
sia by solutions of bicarbonate of lime, while those of the second 
class owe their origin, to the decomposition of magnesian chlorid 
or sulphate by solutions of alkaline bicarbonates. In both cases, 
however, the bicarbonate of magnesia, which the carbonated 
waters generally contain, contributes a more or less important 
part to the generation of the magnesian sediments. The carbon- 
ated alkaline waters of deep-seated springs often contain, as ÍS 
A 53 E; 
E AT es NOA E O ELAN AE a T E ae 
