three Minerals from Lake Superior. 487 
- The mineral is therefore essentially a silicate of lime, in 
which the oxygen of the silicic acid is double that of the 
lime; the formula is therefore Ca? Si*, or that of common 
table-spar. 
The amount of water in the mineral dried at 100° C. was 
found by two determinations to be 2.92 and 2.96 per cent. ; 
still the quantity seems too small to allow it to be considered 
as formfng an essential part of the mineral, especially as it 
retains all its properties unchanged after ignition. 
This mineral forms a large mass nearly afoot in thickness 
in the trap of Kewenaw Point at the Cliff mine, and also at 
Scovill’s Point on Isle Royale. 
It receives a beautiful polish. 
IL. JACKSONITE, 
Associated with the above described mineral there occurs 
at both of the localities above mentioned, a mineral, which 
was found on examination to differ in composition from any 
known silicate ; I have therefore given it the name of Jack- 
sonite, in honor of Dr. C. T. Jackson, whose name is so well 
known in connection with the Lake Superior region, and to 
Whom we are so much indebted for our knowledge of its 
mineralogy. 
The Jacksonite occurs in finely radiated, and lamellar- 
radiated masses of a white color slightly tinged with green. 
Its hardness is 6; specific gravity 2.881; lustre vitreous; 
translucent. 
The finely pulverized mineral is perfectly, though slowly, 
dissolved by chlorohydric acid, the silica separating in the 
form of a flocky powder. 
Before the blowpipe in the platina forceps it fuses very 
readily, with strong intumescence, and emits a brilliant yellow 
ht. 
A large quantity of soda dissolves it readily; if more of 
the assay be added it swells up to an infusible slag. 
It gives with borax a colorless, transparent glass; with salt 
