176 The Arner lean Geologist. March, isgo 
amygdaloidal structure at the base of the slope than higher 
up, but does not carry so much visible copper. The whole ap- 
pearance is that of a dyke filling a fissure in the Animikie 
strata. Whether the amygdaloidal trap cuts the diabase trap 
cap which rests on the shales, cannot at present be determined. 
It probably does not. 
There are, however, some circumstances which, in view of 
the meagerness of the exposures, render other explanations 
possible, and the conclusion that the formation is a dyke can 
only be held tentatively. These are (1) that at a level of 100 ft. 
up the slope there is a small outcrop of a fine-grained, brownish- 
red sandstone such as is common in the Keweenawan series, 
but which has nowhere been observed by the writer in the Ani- 
mikie ; (2) dykes do not usually present a highly vesicular or 
amygdaloidal structure which is rather the characteristic of 
surface flows ; (3) the supposed dykes are much more vesicular 
and the vesicules are larger at the foot of the sloping line of 
outcrop than at the top, which is contrary to what we would ex- 
pect with reference to the formation of vesicules in any molten 
mass of rock, the mass being usually more vesicular toward the 
top where the pressure is less. The other possible explanations, 
none of which are supported by any direct field evidence, are 
( 1 ) that the supposed dyke may be a small piece which has been 
let down within the Animikie by faulting from the Nipigon 
which may be supposed to have once covered the Animikie 
here as it does elsewhere ; or (2) it may be the infilling of a 
fissure in the Animikie formation from above by a surface 
flow of vesicular lava which brought down with it portions of 
the Nipigon sandstone ; or (3), least likely of all, it may be a 
small outlying patch of the Nipigon resting on the surface of 
a pre-existing slope of the Animikie. 
A little exploration and mining would soon set at rest these 
questions as to the precise nature of the formation. We have 
this fact at least,that it is a rock of the same facies as the amygda- 
loids of the Keweenawan or Nipigon, similarly charged with 
copper, well within the Animikie slates and apparently cutting 
them. In this respect the formation differs from the amygda- 
loids of the Keweenawan which are interbedded and contem- 
poraneous with the other rocks of that age, while the forma- 
tion here discussed is apparently of later age than the Ani- 
mikie, and may very probably be of Keweenawan age, though 
associated with Animikie rocks. 
