272 Scientific Intelligence. 



11. MINERALOGY AKD GEOLOGY. 



1. On the prdbahle origin of some Magnesian Rocks ; "by T. Sterky 

 Hunt, (in a letter to one of the Editors.) — It is well known that certain 

 mineral waters when exposed to the air deposit large <|nantities of carbo- 

 nate of lime, and that the beds of travertine thus produced at the present 

 day, often form extensive rock masses. We cannot doubt that some- 

 ancient limestones have had a similar origin, and among these, portions 

 of the sparry limestones of Eaton in the Hudson Ri^'er group, which 

 sometimes exhibit the agatized structure and texture of alabaster. The 

 various deposits of carbonate of lime thus formed, often contain iron as 

 oxyd or as carbonate, besides a great variety of other elements including 

 many of the metals, but magnesia is always wanting in them, or present 

 only in traces. Berzelius remarked in his analysis of the waters and the 

 travertine of the Carlsbad springs, that the large portion of carbonate of 

 magnesia which the water contains, remains dissolved after the precipita- 

 tion of the lime, and is only separated by evaporation. The precipitate 

 produced in a solution of a magnesian saU by the addition of carbonate 

 of soda is redissolved by an excess of the magnesian or the alkaline salt, 

 but the evaporation of the liquid deposits the magnesian carbonate in a 

 granular form. » 



In my investigations of mineral springs of the Lower Silurian Rocks of 

 Canada, I have found certain waters containing common salt with large 

 amounts of chlorid of calcium and magnesium, but destitute of carlK)n- 

 ates; there are other saline waters, which, with less of earthy chlorids, 

 give by evaporation large amounts of earthy carbonates, in which the 

 magnesia prevails, sometimes almost to the exclusion of lime. These 

 springs apparently owe their origin to the admixture of waters of the 

 first-mentioned class with those of the alkaline springs, so common in the 

 same region, and in which carbonate of soda predominates. By the 

 careful addition of carbonate of soda to such a water, nearly the whole 

 of the lime may be precipitated while a great part of the magnesia ~^ 

 mains in solution- The spontaneous evaporation of such magnesian 

 waters arising from the action of carbonate of soda upon sea-water or 

 that of the springs above mentioned, would give rise to beds of magne- 

 sian carbonate, either pure or mingled with carbonate of lime. 



Such deposits occur in Canada, in the form of thin layers, interrupted 

 beds, or lenticular masses, interstratified with pure limestones, which 

 sometimes appear to be travertines, and at others perhaps ordinary sedi- 

 mentary limestones, holding orthoceratites, trilobites and other fossils of 

 the Hudson River group. The magnesian beds are always granular, des- 

 titute of fossils, and generally mixed with sand or clay ; they often pass 

 into a veritable conglomerate, holding fragments of quartz or limestone. 

 ' The carbonate Is sometimes a dolomite, and at others a magnesite, desti- 

 tute of lime, but a portion of carbonate of iron is never wanting. The 

 pure carbonate of lime also, but more rarely, serve as the cement to 

 quartzose sand or limestone pebbles, and the whole of these rocks are 

 interstratified with the sandstones and shales, often graptolitic, which 

 make up the great mass of the Quebec division of the above mentioned 

 group. 



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