REVIEWS. 



377 



equivalents of calcareous spar is heated in sealed tubes to 200 degrees centi- 

 grade, it is completely converted into dolomite and sulphate of lime. 



Marignac, in like manner, found that at 200 degrees centigi'ade, carbonate 

 of Hme, with a solution of chlorid of magnesium, slowly gave rise to a double 

 carbonate of Hme and magnesia ; after six hours the product contained 52*0 per 

 cent, of carbonate of magnesia (Eavre. Bull. Soc. Geol. Prance, vol. vi., p. 318). 



De Senarmont found ui 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 chlorid of calcium 

 present is more than equivalent to the magnesia in. solution ; but at 150 degrees, 

 whether the lime-salt be iu excess or not, a precipitate of carbonate of magnesia 

 is obtained with little or no Hme. 



Taking the experiments of Morlot and the theory of Haidinger as a point 

 of departure, Pavre attempts to explaui the formation of dolomites. He 

 supposes that eruptions of igneous rocks at the bottom of a sea 500 or 600 

 feet in depth would afford the necessary conditions of heat and pressure ; and 

 since the dolomites of the Alps are associated with melaphyres, which are more 

 or less magnesian, he supposes a simultaneous evolution of sulphurous and 

 hydrochloric acids ; these acting upon the ejected rocks, would produce the 

 magnesian salts necessary for the conversion into dolomites of the adjacent 

 Hmestones, which, according to him, are interstratified near their base with 

 pyroxenic tufa. These dolomites of the Tyrol are fiUed with smaU cavities, 

 while they retain the marks of stratification, and exhibit the remains of corals 

 and encrinites. Eavre supposes they were origiuaUy deposited as pure Hme- 

 stones, and became cavernous in their 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 Yon Buch, based in great part upon these dolomites of 

 the Tyrol, supposes that the dolomitization of Hmestones 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 heating together to low redness, in an iron tube, frag- 

 ments of porous Hmestone and anhydrous chlorid of magnesium for some hours. 

 The soluble matter beiug then washed away, the residue effervesced strongly 

 at first with hydrochloric acid; but the action then became feebler, and 

 the residue exhibited transparent crystals under the microscope, which were 

 supposed to be dolomite, but were not further examiaed (Philosophical 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 coimect the dolomitization of limestones with the eruption of 

 igneous rocks. Pournet, too, has since shown that the melaphyres associated 

 with the dolomites of the Tyrol, so far from being intrusive, are themselves 

 stratified rocks, {)robably of the carboniferous age, metamorphosed in situ, and 

 that their 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 contains roUed pebbles of the subjacent melaphyres 

 (Bull. Soc. Geol. de Prance, 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 



