and the formation of Gypsum and Magnesian Rocks. 371 
tained 3:9 p. c. of clay and sand; its solution gave 0°6 p. ¢. of 
oxyd of iron with alumina, but no magnesia. Similar examples 
. of fossils replaced by dolomite oceur in gray limestones associa- 
: with the travertines and dolomites of Pointe Levis (§ 50). 
53. Magnesian limestones are very frequently destitute of or- 
ganic remains; in some cases however they may contain ealea- 
reous fossils, as in the Niagara limestone at Dudswell, where 
corals of the genera Cyathophyllum, Poriles and Favosites, com- 
# | posed of pure carbonate of lime, and generally bluish-black in 
2 color, are imbedded in a yellow ferriferous magnesian limestone 
4 which contains an excess of carbonate of lime. This limestone 
J gave by analysis carbonate of lime 56°60, carbonate of magne- 
J sia 11-76, carbonate of iron 823, insoluble quartz sand 2672= 
; 98°31. The portion soluble in cold dilute acetie acid was car- 
bonate of lime with four per cent of carbonate of magnesia and 
: a trace of iron, and the residue when digested with dilute hydro- 
chloric acid left 52-0 p. ¢. of sand me) pyrites; the dissolved 
part consisting of earbonate of lime 51°75, carbonate of magne- 
a sia 35°73, carbonate of iron 12°52=100-00. : 
a | In the magnesian limestone of Galt in western Canada, whieh 
isa pure crystalline dolomite, there are numerous casts of i 
: valve molluscs, the shells of which were evidently removed by 
solution after they had been filled and enveloped by the dolo- 
i mitic matrix, since the walls of the cavities once occupied by 
the shells of a large bivalve, Megalomus Canadensis, retain the 
markings of the inner and outer surfaces of the shell. Similar 
; moulds of Ophileta compacta are abundant in the blue dolomite 
of Beauharnois, which belongs to the Calciferous sandrock; in 
4 a dolomite of the same logical formation from the Mingan 
a4 
; 
Islands, the shells of Ophileta, Maclurea and Scaphites are ré- 
Plae sili 
n some portions of the Galt formation fragments of encrinal 
columns are found replaced by dolomite, which is only distit- 
guished by a little difference of color from the matrix. It woule 
appear in this case as if the calcareous fossil refi Re first 
Temoved by solution ($30) the cavity had been su agent! 
filled with dolomite as in the casts found in the Ottawa an 
($52). : a 
54. Although dolomites not unfrequently form by ers 
masses of great thickness, as in the Jurassic formation of the 
