366 On some Reactions of the Salts of Lime and Magnesia, 
belong to the first division mentioned above. These modes of 
formation however do not account for an important fact in the 
history of most stratified gypsums, which is that of their almost 
constant association with carbonate of magnesia generally in the 
orm of magnesian limestone. Beds of dolomite are often inter- 
stratified with or include beds or masses of gypsum, while dolo- 
mite and carbonate of magnesia are sometimes found imbedded 
in gypsum or anhydrite. For a description of the magnesite 
which is disseminated in the gypsum of Salzburg, see Dufré- 
noy, Minéralogie, 2d ed., ii, 424. Small masses of compact 
and crystalline gypsum, occasionally associated with crystals of 
ealcite and quartz, abound in some of the dolomite beds of the 
so-called Calciferous sandrock in Canada, and crystallized gyp- 
sum and anhydrite, together with sulphates of baryta and stron- 
tia, and fluor spar, occur in geodes in the magnesian limestone 
of Niagara. The anhydrous sulphate of lime not only forms 
beds by itself but is often met with disseminated in masses, 
grains or crystals through beds of gypsum, and even interstrati- 
fied with it, as in the south of France, in the Hartz, Switzerland, 
and in Nova Scotia, as described by Mr. Dawson. (Acadian Ge- 
ology, 225.) The conversion of beds of anhydrite into gypsum 
by the absorption of water, and the attendant phenomena, have 
been described by Charpentier. 
45. Both the hydrous and anhydrous sulphate sometimes form 
the cement of conglomerates or breccias, which enclose flints, 
fragments of shale and of limestone, as at Pomarance in Tuscany, 
(Scarabelli, Bull. Soc. Géol. de France, [2], xi, 846,) and also at 
Bex, where the cement of the conglomerate is a granular anhy- 
drite (Charpentier, Jbid., [2], xii, 546). : 
ums moreover often include clay and sand, and sometimes 
contain a considerable admixture of carbonate of lime, which 
in those of Aix, according to Coquand, amounts to eight per 
cent. The gypsums of Montmartre also contain, according to 
Delesse, besides some clay and sand, and several hundredths of 
carbonate of lime, not less than three per cent of soluble silica 
intermixed. Silica in the form of flint or chert is sometimes 
found in concretions with gypsum; thus in the miocene clays 
near Bologna in Italy, flints are met with associated with sul- 
phates of lime, of baryta and strontia, together with pyrites and 
ulphur. Masses of sulphate of strontia are likewise found in 
clays with the gypsums of Montmartre, and the association 0 
sulphate of strontia with the sulphur, m and rock salt of 
Sicily is well known. The gypsums of Madrid, which occur 10 
ey are according to Casiano de Prado, accompanied 
by ees chert and of magnesite (Bull. Soc. Géol. de France, [2]; 
aPC ee 
