50 ANNUAL ADDRESS, MDCCCLXXIII. 



from water ; in fact, veritable travertines. Interstratified 

 with these travertines, however, there are beds of fine 

 granular opaque limestones, weathering bluish-grey, and 

 holding in abundance remains of orthoceratites, trilobites, 

 and other fossils, which are replaced by a yellow-weather- 

 ing dolomite." 



"The facts which we have indicated clearly show that 

 the dolomites just described have been precipitated from 

 water, under conditions which brought more or less sand 

 and clay, and sometimes fragments of the adjacent rocks, 

 into the basins where this process was going on, during 

 the intervals of which the travertines and fossiliferous 

 limestones were deposited, to the exclusion of magnesia. 

 Similar conditions are met with in some limestones of the 

 Niagara division, in the eastern basin, where purely cal- 

 careous corals, of many species, are embedded in a paste 

 of granular magnesian carbonate of lime, which would 

 seem to have been precipitated in the medium where the 

 zoophytes grew. A somewhat different process is presented 

 in the replacement, by dolomite, of fossils in the limestone 

 at Pointe Levis, as well as many localities in the Chazy 

 limestone, where various shells are replaced, and some- 

 times entirely filled, with a crystalline ferriferous dolomite, 

 the surrounding limestone being destitute of magnesia and 

 iron. It is known that those mineral waters which hold 

 large quantities of carbonate of lime and magnesia in 

 solution deposit only the lime on exposure to the air, and 

 retain all the magnesia in solution : hence travertines and 

 tufas, both ancient and modern, contain little or no mag- 

 nesia. The carbonate of this base is soluble to a con- 

 siderable extent, in solutions both of magnesian and 

 alkaline salts, but is deposited when those solutions are 

 boiled, or evaporated at low temperatures. Thus the 



