ORIGIN. DD 
sreat extent and importance, but have no special bearing on the subjects 
of this paper, and so need no extended description. It is probable that 
some rocks mapped as agelomerates are really autoclastic, especially those 
including fragments sharply angular in form and all of the same kind. 
These aneular blocks of rock could hardly have been hurled into the air 
as bombs. Lawson mentions transitions between agglomerates and true 
conglomerates with waterworn pebbles, which might readily occur where 
volcanic materials drop into the sea. 
The autoclastics are not very extensive and will merely be mentioned. 
They are produced by the action of crushing and shearing forces in rocks 
above the level where pressure produces plasticity, and so give a hint as 
to the original depth of the rock beneath the surface. The best exam- 
ples known to me, omitting the eruptives already mentioned, are the 
limestone breccias of Steep Rock lake and certain conglomerates to be 
seen a mile west of Fort Frances, where in the sharp folding of thin beds 
of sandstone between layers of schist the latter has yielded plastically, 
while the former was broken to fragments now embedded as pebbles in 
the schist. 
The ordinary clastics, true sediments, are of special interest as giving 
clear ideas of the conditions of the time, and so will be described more 
at length than the two previous groups. The Keewatin, though very 
largely of eruptive origin, contains important sedimentary members, 
and the Couchiching is wholly sedimentary. 
KEEWATIN ROCKS 
The water-formed clastics of the Keewatin are of great variety, includ- 
ing limestones, slates, quartzites, grits, graywackes, breccias, and pebble 
and boulder conglomerates. The limestones are, however, of limited 
extent, being found in any thickness only at Steep Rock lake, 70 miles 
east of Rainy lake, where there is a small area differing both petrograph- 
ically and structurally from the rest of the region. These limestones 
have a very modern look, being scarcely at all crystalline in appearance, 
haying cherty layers in gray limestone at some points and black, very 
carbonaceous beds at others. One almost expects to discover fossils in 
them, but none have been found. They have been folded in an extraor- 
dinary way into an anticline and syncline having their axis inclined 60 
or more degrees.* 
The slates are widespread, passing often into phyllites. Many of them 
contain carbonaceous matter, and some examples have a graphitic look 
and soil the fingers. One of them, from an analysis by Dr Adams, was 
*H, L. Smyth: Geology of Steep Rock lake, Am, Jour. Sci., vol, xlii, 1891, pp, 317-331, 
. 
