T. S. Hunt on Lime and Magnesia Salts. 67 
$110. Carbonic-acid water may be employed arsed = 
(52°0 per cent), the action of 500 ¢.c. of water saturated with 
yaar acid during two and a half hours, removed from one 
containing only 48°5 per cent of magnesian 
teetes The residue, from which the more finely divided 
portions had thus been removed, was very slowly see tr by a 
‘solution of carbonic acid, a second portion of 500 ¢.¢. of whic 
after four hours, took up 0: 145, and a third portion, after eighteen 
hours more, 0°162 gr. of the two carbonates, in both cases con- ’ 
ee of earbonate of lime 53-0, carbonate of 1 magnesia 47:0. 
111. In coneluding this part of the subject it is to be ree 
marked that two things in the history of dolomite may be 
regarded as established: first, its origin in nature by direct 
a er and not by the alteration of non-magnesian lime- 
stones; and second, its artificial production by the “direct union 
of mixtures of the carbonates of lime and magnesia at tempera- 
tures above 120° C. e question next arises ; whether all dolo- 
mite strata have been exposed to such a temperature, or whether 
there are yet unknown conditions under he ich the double 
carbonate can be found at lower temperature 
magnesian limestone from the elevated coral island of 
The 
. Matea, described by Dana (this Jour., I xiv, 82), is, according 
to the analysis of Silliman, and my ow n subseque ent examination 
and analysis (Ibid., [2], xix, 429), a true dolomite with a slight 
excess of carbonate of lime, and is is regarded by Dana as of 
recent origin, and as derived, in some way, from the alteration 
of coral mud. If this origin ‘be established beyond a doubt, it 
is to be remarked that the separation of carbonate of magnesia 
from sea-water requires peculiar conditions, which evidently are 
rarely fulfilled in the case of these coral deposits; and its pro- 
duction being conceded, the volcanic agencies so active in these 
regions may have ver. well furnished the heat requisite to form 
dolomite before the elevation of the island. 
§112. Apart from the formation of stratified sedimentary 
dolomite, we have also to keep in mind the frequent occurrence 
A this double carbonate as a mineral of secondary bee ap atts: 
ning drusy cavities, filling veins, and even the moulds of foss 
shat ($ 52, 53). The conditions of its aah from cate 
waters are ‘probabl not unlike those of the quartz, fluor, and 
pi iy with ee in its form of bitter-spar, it is often 
and as subjects for farther investigation, may yet 
oe more i ht on the agencies which have effected the union 
and crystallization of the two carbonates in sedimentary 
Montreal, Jan. 1866. 
