and the formation of Gypsum and Magnesian Rocks. 367 
gy rm 
associated clays. (Bischof, Chem. Geology, ii, 421-481.) pa 2 
deposits of both of these sulphates occur with gypsum and rock- 
46. The gypsums of the class which we are now describin 
appear in every geological period. To these apparently belong 
the masses of gypsum and anhydrite, which at Fahlun are asso- 
ciated with dolomite and serpentine in the chloritie bands of the 
‘oldest crystalline rocks of Scandinavia, the probable equivalents 
of the Laurentian system of North America. On this continent 
the oldest known gypsums are those already mentioned as occur- 
ring near the base of the paleeozoic series, and in what is called 
by the geologists of New York the Caleiferous sandrock. \ 
ascend the series gypsum is occasionally met with in the Clinton 
and Niagara groups, until we reach the Onondaga salt-group in 
the Upper Silurian rocks of Canada and New York, which con- 
tains great deposits of dolomite and gypsum, occasionally accom- 
panied by sulphur. The gypsums, anhydrites, and brine springs 
of Nova Scotia belong to the Carboniferous series, while the fre- 
quent recurrence of gypsum in Europe through all the higher 
rocks up to the Miocene inclusive, is too well known to require 
Notice. 
_ 47. The so-called primitive gypsums and anhydrites, which 
in the Alps and Pyrennees occur interstratified with crystalline 
— and alters both fossiliferous limestone and gypsum. The latte 
becomes mingled with and finally passes ito limestone. (Co- 
quand, Bull. Soc. Géol, de France, [1], xii, 345.) | In Al 
h m is associated with crystalline | 
eryl are found 
the um, 
| Hartz, a =: ‘3 , con- 
n nodules of a silicate of magnesia col yy carbonaceous 
. crite the sci and the chemical composition of 
[2], iv, 882.) 
