MINERALOGY OF BLACK LAKE AREA. 
47 
six-sided flakes of clinochlore. Within the cavities, where the garnet 
has had an opportunity to crystallize freely, fine crystals occur plentifully, 
some of which attain a diameter of half a centimetre. In habit they are 
always dodecahedral ; occasionally this form occurs alone, but as a general 
rule its edges are truncated by narrow faces of the trapezohedron n(211). 
In addition to these forms, the four-faced cube 6(610) and the trape- 
zohedron m(3.11) were observed on one of the crystals measured; but in 
each case these latter forms were present only as minute facets, which 
did not yield bright signals. 
An analysis of these crystals gave the results shown in the last 
column of the following table : 
Analyses of Grossidarite. 
Theoretical 
composi- 
tion 
1 
Wakefield 
(white) 
3 
Hull 
(colourless) 
4 
Orford 
(yellowish- 
white) 
5 
Southwark 
pit, Black 
lake 
Molecular 
ratio 
SiO s . ...... 
40-00 
38-80 
39-85 
38-60 
39*49 
■658 
. .3 
AlgOs. . . . ... 
22-70 
22-66 
22-07 
22-71 
22*35 
•219 
..1 
F6jOfl. ........ 
1-75 
1*13 
FeO.'. 
1-60 
1-00 
•014] 
CaO. ........ 
37-30 
35-00 
36-31 
34-83 
36*62 
•654 
'. .3 
MgO. 
0-68 
0-68 
0-49 
0-28 
•007 
MnO. 
0-30 
0-15 
•002 
K*0 
0-47 
H s O. ...... . 
1-10 
Sp. gravity 
Refractive 
index. 
100-00 
99-19 
3-525 
1 »7438 2 
100-04 
99-80 
3-52-3-53 
99-89 
3-60 
1-734 
1. Bullman, Am. Jour. Sc., 27, 1884, p. 306. 
2. Wulfing, 
3. Priv. com. M. D. Munn (cited by Dana, p. 440). 
4. Hunt, Sterry, Geol. of Can., 1863, p. 496. (The 1*60 FeO includes some MnO). 
5. Graham. 
The composition of colourless garnet from Hull and of white garnet 
from Wakefield and from Orford are given for comparison. 
The refractive index was determined by the minimum deviation 
method using sodium light and taking the dodecahedron faces for the 
prism. 
Hall Chrome Pit . Specimens of grossularite were collected from the 
Hall chrome pit by J. Obalski 1 many years ago, and are now in the 
mineral collections of the Geological Survey. Many specimens from this 
pit were collected by the writers. The crystals have an amber colour 
i Geol. Sutv., Can., Ann. Rept., vol. X, 1897, p. 123A. 
4 
