Mr. T. S. Hunt on the Origin of some Magnesian Rocks. 379 



carbonic acid is much less than is required to form bicarbonates 

 with the soda, hme and magnesia which these waters contain, so that 

 the magnesia must be held dissolved as a mono-carbonate. In the 

 water of Chambly, on the contrary, there is no deficiency of carbonic 

 acid, and the bases exist as bicarbonates. The temperatures of these 

 springs range from 46° to .53° F. ; some of them are therefore to be 

 regarded as slightly thermal. 



Interstratified with the shales and sandstones of the Quebec divi- 

 sion of the Lower Silurian rocks, which immediately overlie the 

 strata yielding these alkaline waters, are found thick beds of pure 

 limestone, sometimes presenting the agatized structure and semi- 

 translucency which characterize certain travertines, but at other 

 times opake, homogeneous, and including remains of orthoceratites, 

 trilobites and other fossils, xlssociated with these beds of pure car- 

 bonate of lime are others which are magnesian, and contain con- 

 siderable quantities of carbonate of iron, which causes them to 

 weather reddish brown. These beds are always granular in texture, 

 and contain a variable portion of siliceous sand ; they often become 

 conglomerate, enclosing pebbles of quartz and schist, or more fre- 

 quently fragments of a pure compact limestone, seemingly identical 

 with that of the beds just described. Thin layers of the ferruginous 

 magnesian rock sometimes separate beds of the pure carbonate of lime, 

 or form lenticular masses in its midst, and seem to replace its fossils. 

 The pure limestones also sometimes form the base of a conglomerate, 

 or are mixed with sand and argillaceous matters. 



These magnesian rocks, like the pure limestones of this formation, 

 occur in irregular and interrupted beds ; they often attain a thick- 

 ness of many yards, are destitute of fossils, and contain from ten to 

 forty per cent., and even more, of sand or clay. The portion soluble 

 in acids is sometimes a dolomite with carbonate of iron ; at other 

 times the lime is wanting, or present only in traces, and we have a 

 ferruginous magnesite. In two previous notes presented to the 

 Society, I have already explained the manner in which I suppose 

 these siliceous carbonates to have been, in some parts of the forma- 

 tion, transformed into silicates, such as serpentine, talc, chlorite and 

 pyroxene, by the subsequent intervention of heated solutions of alka- 

 line carbonates. 



It appears to me that we may explain the origin of these magne- 

 sian deposits, by the spontaneous evaporation of magnesian waters. 

 If the waters of Carlsbad were to become stagnant above their de- 

 posited travertine, they would yield by evaporation beds of ferru- 

 ginous dolomite, and waters like those of Caxton, Plantagenet, and 

 Sainte-Gencvicve would furnish carbonate of magnesia nearly free 

 from lime. Nothing forbids us to suppose the existence of waters 

 more highly cliarged than these with magnesian carbonate, formed 

 perhaps by the action of carbonate of soda upon lagoons of sea- 

 water, wliose Hire may be removed as carbonate, or by previous eva- 

 poration as sulj)hate. The lagoons in Bessarabia, supplied with the 

 waters of the Black Sea, dc])osit annually large beds of rock-salt ; 

 and it would rccpiire only tlie intervention of waters like those of the 



