eee ates : og wv 
and the formation of Gypsum and Magnesian Rocks. 373 
is well known to consist of equivalents of carbonate of lime and 
magnesia corresponding to 45°65 parts of the one to 54°35 of the 
other, and many magnesian limestones have this composition, or 
contain beside only mechanical impurities, such as sand and 
clay. Others with an excess of carbonate of lime are shown by 
the method of Karsten to be mixtures of dolomite with carbon- 
ate of lime, which is readily separated by the solvent action of 
cold dilute acetic acid ($28, $53). The same chemist however 
found in clefts and fissures of the gypsiferous rocks of Luneberg 
and elsewhere, carbonates of lime and magnesia mingled with 
clay, from which dilute acetic or muriatic acid removed the 
Whole of the lime, leaving a residue of from 40 to 68°0 p. c. of 
ouble carbonate. The free carbonate of lime which they con- 
tain is however probably epigenic and produced by the decom- 
position of a portion of the magnesite by the infiltration of dis- 
solved gypsum. - 
Carbonate of iron often replaces a part of the magnesian car- 
bonate in dolomites, which also sometimes contain carbonate of 
manganese, and even carbonates of zinc, cobalt and lead. It 
hot unfrequently happens that the sum of the other carbonates 
in these ferruginous dolomites is more than equivalent to the 
carbonate of lime. Such is the case with the dolomitic conglom- 
erate of St. Helens (§ 50). : 
The dolomites of the Hudson River group in eastern Canada 
manganese. A grayish granular dolomite from Sutton, 
which contains disseminated chlorite and crystals of —— 
weathers blackish-brown from the presence of man e 
oreign minerals are arranged in bands, and layers of the dolo- 
Mite an inch or two in thickness are apparently ad- 
lime 40-10, carbonate of magnesia 20-20, carbonate of iron 10°65, 
ds of manganese and portions of Report, 
1668 oe 474, and rear enme [2], xxvi, 238.) has e@ manner noticed 
the oceurrence of a proportion of ‘yd of manganese in the olive colored 
slates of the oa series in Pennsylvania, and to the decomposition of 
