128 THE ROYAL SOCIETY OF CANADA 
Thousand Islands district under the name of the Picton granite, by 
Professor H. P. Cushing.} 
Erosion Interval. A long interval of erosion must have taken 
place between the intrusion of the Roberval and the beginning of 
Ordovician sedimentation. The earliest sediments of this time. 
Trenton limestones, were deposited on coarse plutonic rocks, which 
have a surface relief comparable to that of the Pre-Cambrian of the 
present day. Yet they show little, if any, surface decomposition, 
or erosional detritus. A few well rounded pebbles of granite or quartz 
are occasionally found in the lower beds of limestone. But they are 
rare, and nowhere form a definite basal conglomerate. 
Pre-Ordovician erosion appears to have been followed, or accom- 
panied, by very complete removal of erosional débris. 
Ordovician. On this surface Trenton, Utica and Richmond 
sediments were deposited in comformable succession. No later 
Paleozoic strata have yet been found. In some period after Ordovi- 
cian time, perhaps early Carboniferous, extensive faulting took place 
by which a large area was lowered several hundred feet below the 
general level of the region and the basin of lake St. John was event- 
ually formed. 
STRUCTURE OF THE BASIN. 
In places where Pre-Cambrian rocks underlie both the highland 
and the adjacent basin, the escarpment suggests that it is due to 
faulting. This view is fully confirmed by the structure in places 
where the Ordovician strata of the basin are seen in contact with the 
Pre-Cambrian rocks of the surrounding highland. In such places 
the Ordovician, Trenton limestone, is tilted to a high angle, or broken 
and crushed into a confused mass. This deformation of the Ordovi- 
cian at its margin is the more conspicuous as the rocks of this system 
show very little disturbance elsewhere in the district. (See Plate IT, 
B.) The angle of dip in these sedimentaries is usually less than 6°, 
but wherever they can be seen around the margin of the basin there is 
a zone of 100 to 150 yards in width in which the dip is increased to 
25° or 50°, generally accompanied by fracturing and displacement 
of blocks in some part of the zone. Good examples of the fault con- 
tact may be seen immediately south of the railway station of Cham- 
bord Junction; or along the lake shore in range 1 of the township of 
Metabetchouan, between lots 30 and 40; or near Ouiatchouan falls | 
in range 2, lot 22 of the township of Charlevoix. In all cases the 
faults appear to be normal, or gravity faults. 
1Geology of the Thousand Islands Region, N.Y., State Museum Bulletin 
No. 145. 
