HEVIEWS. 
377 
equivalents of calcareous spar is lieated in sealed tubes to 200 degrees centi- 
grade, it is completely converted into dolomite and sulphate of lime. 
Mariguac, in like manner, found that at 200 degrees centigrade, carbonate 
of lime, with a solution of chlorid of magnesium, slowly gave rise to a double 
carbonate of lime and magnesia ; after six hours the product contained 52'0 per 
cent, of carbonate of magnesia (Favre. Bull. Soc. Geol. France, vol. vi., p. 318). 
De Senarmont found in some experiments with mingled solutions of bi- 
carbonate of magnesia and chlorid of calcium, that at the ordinary temperature, 
and at temperatures below 100 degrees centigrade, a precipitate of pure car- 
bonate of hme separates, provided that the proportion of cldorid of calcium 
present is more than equivalent to the magnesia m solution; but at 150 degrees, 
whether the lime-salt be in excess or not, a precipitate of carbonate of magnesia 
is obtained with little or no lime. 
Taking the experiments of Morlot and the theory of Haidinger as a point 
of departure, Favre attempts to explain the formation of dolomites. He 
supposes that eruptions of igneous rocks at the bottom of a sea 500 or 600 
feet in depth would afl'ord the necessary conditions of heat and pressure; and 
since the dolomites of the Alps are associated with melaphyres, which are more 
or less maguesian, he svipposes a simultaneous evolution of sulphurous and 
hydrochloric acids ; these acting upon the ejected rocks, would produce the 
maguesian salts necessary for the conversion hito dolomites of the adjacent 
limestones, which, according to Mm, are interstratitied near their base with 
pyroxenic tufa. These dolomites of the Tyrol are filled with small cavities, 
while they retam the marks of stratification, and exhibit the remains of corals 
and eneruutes. Favre supposes they were originally deposited as pure lime- 
stones, and became cavernous in then- subsequent conversion into dolomite ; 
and he conceives that the sea, beneath which the volcanic eruptions took place, 
was widely extended, and thus explains the formation of dolomites far away 
from any intrusive rocks ; at the same time he admits that compact dolomites 
in many stratified rocks have been originally deposited as such, and are not the 
result of alteration. 
The famous theory of Von Buch, based in great part upon these dolomites of 
the Tyrol, supposes that the dolomitization of limestones has been effected by the 
intervention of some volatile compound of magnesia evolved during the erup- 
tion of the porphyries of that region. In support of this hypothesis Durocher 
made the experiment of heatuig together to low redness, in an iron tube, frag- 
ments of porous Umestone and anliydi'ous chlorid of magnesium for some hours. 
The soluble matter being then washed away, the residue effervesced strongly 
at first with hydrochloric acid; but the action then became feebler, and 
the residue exliibited transparent crystals under the microscope, which were 
supposed to be dolomite, but were not fui'ther examined (Plulosopliical Maga- 
zine, vol. ii., p. 504). 
To Von Buch's theory it must be objected that no known compound of 
magnesium is volatile ; and that it is only by the intervention of water that 
we can at all connect the dolomitization of limestones with the eruption of 
igneous rocks. Fournet, too, has since shown that the melaphyres associated 
with the dolomites of the Tyrol, so far from bemg intrusive, are themselves 
stratified rocks, probably of the carboniferous age, metamorphosed in situ, and 
that theii- alteration was effected long before the deposition of the dolomites, 
which are of the Jurassic period ; for between those metamorphic strata and 
the dolomites are beds of unaltered Triassic rocks, including the Muschelkalk 
and a conglomerate which contauis rolled pebbles of the subjacent melaphyres 
(Bull. Soc. Geol. de France, vol. vi., p. 506). 
Delesse has remarked that in many instances limestones which have been 
regarded as dolomitized by the proximity of igneous rocks, have been rendered 
