) 
the Tilly Foster Iron Mines. 218 
Analysis of red variety gives— 
I Il. Mean. Oxygen. 
SiO, she 35°42 18°86 
FeO 9°7 9°70 9°73 2°14 
MgO 51°70 52°07 51°88 20°75 
iT 5°3 5°42 5°38 rose 
102°41 
Oxygen replaced by fluorine, 2°26 
100°15 
The brown variety gives 7°8 Mg and 3Si, in which part of the 
oxygen of magnesia is replaced by fluorine, or MgFl, +3Mg? Si. 
1. 
The fluorine on the above analyses was determined by the 
he silica and 
. A. Burt made an analysis of a dolomite found at 
this locality, obtaining— 
is Il. Mean. Oxygen. 
‘91 “49 . . 
MnO 13 64 39 07 
30°30 29°98 30°14 8°61 
MgO 20°78 20°80 20°79 8°31 
CO, 46°97 47°05 47°01 10°02 
99°09 98°96 99°03 
This corresponds to FeCo, 1°13, MnCo, 0°63, CaCo, 53°82, MgCo, 
43°66 99-24, 
The ratio of CaCo, and MgCo, is 1: 1 nearly. 
Besides the minerals mentioned above, there occur at this 
locality pyrite, chalcopyrite, pyrrhotite, calcite, quartz (small 
crystals), a few specimens of fluorite and apatite. I found also 
two specimens of molybdenite. : ee 
I gladly take this opportunity of expressing the obligation 
which I am under to Prof. Allen for his kindness in furnish- 
ing me with material for my work and advice during the pro- 
gress of it. 
Sheffield Laboratory, New Haven, Ct., May, 1873. 
ay Fresenius’s Quantitative Analysis, American edition, p. 404; Zeitschr. Analyt 
em.,v, 190. Pogg. Ann., liii (1841), pp. 130-9. 
