Association of American Geologists and Naturalists. 141 
zation, by the action of igneous magnesian rocks. In some of the 
localities discovered by that distincuished geologist, thick beds of non- 
magnesian limestone existed between the dolomite and the igneous 
rocks, from which the magnesia was supposed to have been exuded. 
Von Buch supposed in such cases, that the pyroxenic porphyry pene- 
trated the central mass at some point which was not visible, and thus 
conveyed magnesia to the superincumbent carbonate of lime. Dr. J. 
thought it more probable that the limestone, so situated, had become 
charged with carbonate of magnesia, by means of water charged with 
bicarbonate of magnesia; for the carbonate of magnesia would be de- 
posited only at the surface, where its solution was freed from pressure. 
Heat, by expelling one equivalent of carbonic acid, would also cause a 
deposit of carbonate of magnesia from a solution of the bicarbonate. 
‘Tn the instances cited by Mr. Dana, if no magnesian springs occur 
at the present time, we may reasonably suppose their former existence 
as one of the effects of volcanic action during the semi-extinet state of 
volcanoes. We may conceive of the disengagement of carbonic acid 
from various carbonates, acted upon by chlorohydric acid, or by sul- 
phurous acid, both of which are abundantly exhaled from volcanic 
vents. If then carbonic acid gas was disengaged and discharged 
through comminuted volcanic magnesian rocks, such as tuffs, volcanic 
ashes and various pyroxenic rocks; those substances would be decom- 
posed and their magnesia would be dissolved under pressure of the 
ocean, by carbonic acid, and would form bicarbonates, which would 
deposit the carbonate of magnesia the moment the solution was freed 
from pressure, or was acted upon by heat. Hence the various pulve- 
rulent carbonates of lime, corals, &c. might in this manner be charged 
with carbonate of magnesia. It is possible thus to account for the form- 
ation not only of the compact secondary magnesian limestones, but 
even for the formation of granular dolomite, but it is probable that the 
latter variety was rendered crystalline by the subsequent action of heat. 
Mr. Dana said, in answer to Dr. Jackson’s suggestion, that 
there existed no springs of hot water charged with carbonic acid 
or bicarbonate of magnesia, as Dr. Jackson had supposed, in any 
of the coral islands of the Pacific, and therefore such an explana- 
tion must rest entirely on hypothesis, and in reply to an enquiry 
from the Chair, he further stated that so far as Mr. Silliman had 
examined the corals brought home by him from the Pacific, they 
had. proved to be pure carbonate of lime, but thus far only a few 
had been analyzed, not enough to be the basis of an opinion, 
as to the presence or absence of magnesia in them. He consid- 
ered that the carbonate of lime was secreted by the powers of 
