164 The American Geologist. March, 1891 
On the south side of Rainy river opposite Sec. 20, Tp. 5 S., 
R. XXVIII of the Canadian township survey, a dyke was ob- 
served having a width of from 150 to 200 feet and cutting horn- 
blende schists with a north-northwest strike. No specimen was here 
obtained at the immediate contact, and in one a little removed 
from the contact the porphyritic structure was only represented by 
blebs of polysomatic augite imbedded in an ophitic base which 
approached in texture that of the specimens taken at four or six 
feet from the contact in other dykes. The ophitic structure pre- 
vailed in two other specimens, one taken at six feet from the con- 
tact and one from the middle. Quartz was observed in both of 
these but not in the first. The percentage of silica and specific 
gravity of the first and third specimens is as follows : 
SiO, 
Near 
Contact. 
49.81 
Sp. g. 3.221 
Middle. 
50.10 
3.008 
Series of specimens from several other dykes were also ex- 
amined, but the limit of space will not permit of further detailed 
descriptions. Generally, however, it ma} T be said that the por- 
phyrite structure almost invariably characterizes the dyke rock at 
the contact and that this rapidly grades into an ophitic structure 
• which in turn appears to grade very gradually into the granular 
structure. The latter, it must be said, is developed to the entire 
exclusion of the ophitic structure only in a few of the cases ob- 
served. The increasing proportion of quartz toward the middle 
of the dj'kes is a very constant character. In one dyke, namely 
that on the south side of Rainy river opposite the town of Fort 
Frances, well defined crystals of enstatite were observed in the rock 
at the contact as a porphyritic constituent while none of this 
mineral was observed in other parts of the dyke. 
NOTES ON THE GEOLOGY OF SOUTHWESTERN 
NEW YORK. 
By Gilbert D. Harris. 
During the early part of the present season, a well was sunk at 
Jamestown, N. Y. , to a depth of 3263 feet. For the proprietors, 
the undertaking was somewhat unfortunate, since neither oil nor 
