Latest Eriiptlres of Laic t Superior Refilon. — WInchclt. 273 
3. There are uo pyroclastic rocks associated with them. 
4. They are never glassy. 
5. They are never amygdaloidal. 
6. They exhiljit no flow structure. 
7. They have no ropy or wrinkled surfaces. 
8. They have no lava-breccia associated with them. 
9. They came in contact with the slates after the latter wei-e hard 
and brittle, and had acquired their cleavage: yet they never repose upon 
a surface which has been exposed to sub-aerial weathering. 
They are ititriisive because: 
1. They are strictly analogous to the great dikes of the region, (a) In 
their general relations to the adjacent rocks, and in their field aspect. 
(6) In that both their upper and lower sides of the sheets have the 
facies of a dense aphanitic rock, which grades toward the middle into 
a coarsely crystalline rock. 
2. They have a practically uniform thickness over large areas. 
3. The columnar structure extends from lower surface to upper sur- 
face, as it does from wall to wall in dikes. 
i. They intersected the strata above and below them after the latter 
had been hard and brittle. 
5. They may be oVjserved in direct continuity with dikes. 
6. They pass from one horizon to another. 
8. The bottom of the sedimentary strata above them, wherever it is 
observable, is a freshly ruptured surface. 
9. Apophyses of the trap pass from the main sheet into the cracks of 
the slate above and below. 
10. The trap sheets, particularly at the upper contact, hold included 
fragments of the overlying slates. 
11. They locally alter the slates above and below them. 
An intrusive rock may appear at the surface and become a 
lava flow at other places. The writer has been unable to find 
an}^ description of the individual localities of these diabases, 
in the region here considered, which mentions any amygda- 
loidal structure or other characters of eruptive surface rocks. 
Robert Bell, however, in his summary section of the rocks of 
Thunder and Nipigon bays,* mentions layers of trap between 
conglomerates and sandstones, most)}' of a light color, which 
are "often amygdaloidal," occurring on the east side of Black 
bay. In the special descrii)tion. however, of the east side of 
Black bay such characters are not mentioned and it is left to 
be inferred that in making up tlie generalized section this 
character was added as one of the usual features of the Ke- 
weenawan traps, based on what the author knew of those 
traps in general, since he describes these sheets uniforml}'- as 
♦Geological Survey of Canada, Report for 1866-69, p. 320. 
