372 On some Relations of the Salis of Lime and Magnesia, 
The magnesian layers being pulverulent, the encrinal columns, 
which are pure carbonate of lime, are easily separated from their 
matrix, which gave me carbonate of lime 40°95, carbonate of 
magnesia 24:19, carbonate of iron 27-03, silicious sand without 
umina 9°01=101-18; the iron was in part as peroxyd. ‘The 
bluish crystalline limestone distant an inch from the magnesian 
layer gave 18:4 p. c. of white insoluble residue and 1-09 p. ¢. of 
carbonate of magnesia. 
Tn these strata we sometimes meet with similar reddish pul- 
verulent layers which contain no carbonate of magnesia, but are 
composed of carbonate of lime with a large amount of peroxyd 
of iron; such a mixture in one instance forms the cement of a 
breccia of fragments of the blue limestone; it was perhaps at 
one time a double carbonate of lime and iron. 
The thin beds of dolomite above described are closely asso- 
ciated with those holding the dolomitic casts of orthoceratites 
eady noticed; these were enclosed in a nearly black compact 
limestone, which during its solution in hydrochloric acid evolved 
traces of sulphuretted hydrogen. The residue contained a little 
iron pyrites which was removed by nitric acid; it was black 
from carbonaceous matter, but became white by ignition in the 
air, and was an impalpable powder, equal to 12°8 p.c. of the 
rock. Dilute soda ley removed from it 9° p. ¢. of its weight of 
soluble silica, and the residue had nearly the composition of a 
feldspar. It gave me, silica 73°02, alumina 18°31, lime 0°93, 
magnesia 0°87, potash 5:55, soda 0°89=99'57. 
The fossiliferous yellow magnesian limestones of Dudswell 
(§ 53) are in like manner interstratified with beds of gray crys- 
cur interstratified both with limestones of organic origin and 
i i i i its. Allied to 
St. Maxence in France.—(See Damour, 
[2], xiii, 67.) ao. E 
chemical constitution of the rocks containing carbon- 
now de: our consideration. Pure dolomite — 
