104 'J^he American Geologist. February, i9.)5 
changed. Those of mellilite are changed as before. The 
oHvine phenocrysts are double the size of the others and are 
sharp as needles, except that they enclose blebs of the 
ground. , Tlieir decomposition is peculiar. They are some- 
times a homogenous matted mass of serpentine fibres lacking 
the common decomposition along fissures. They are gener- 
ally swarming full of the prisms of the titanaugite described 
above, in radiating tufts, or growing into the interior of the 
fibrous mass from the surface. Sometimes they form a loose 
network through the whole. They are sharp, square prisms, 
bounded by the pinacoid faces. 
The specimen had a greasy label in Hall's cramped hand : 
"Found at Is-se-luk-ju-nor. Between Frobisher Bay and 
'Rescue Bay,' on the land as we pass from head of latter to 
the inlet tliat makes up within 2]^ miles. (This route is the 
one in winter in passing between Oo-pung-ne-wing and 
Rescue Harbor.) Given me by Kou-isse." 
This was doubtless at Bayard Taylor pass. See Hall's 
Arctic Research Expedition ; map. 
The age of these eruptives cannot be told. They are. 
associated with a large range of Archaean rocks — granites, 
granitvtes granulytes, magnitite gneisses, coccalite lime- 
stones containing ruby spinels, but adjacent, are probably 
Carboniferous rocks, and certainly highly fossiHferous Lower 
Silurian rocks from which in the article cited I described a 
large number of fossils. 
MONTANA GYPSUM DEPOSITS. 
By Jesse Perky Rowk, University of Montana, Missoula. 
PLATEtS VII, VIII, IX, X. 
Little has been written concerning the gypsum deposits 
of Montana, as the discovery and economic development of 
the beds in the state date back but a few years. Perhaps 
the first commercial use made of native gypsum in Montana 
was in the year 1894 — and the first discovery the year pre- 
vious. The state, however, can boast of as large gypsum 
