6 
MUSEUM BULLETIN NO. 8. 
Bruce Area. 
A complex of hornblende-schist, schistose amygdaloidal 
lava, and other basic igneous materials, exposed near Thessalon, 
constitute the oldest rock group in this area. It is intruded 
by granite-gneiss belonging to a batholithic mass that extends 
far to the east along Lake Huron. These schists and granite- 
gneiss form a crystalline basement upon which reposes, in con- 
spicuous unconformity, a mantle of conglomerate, quartzite, 
greywacke, and limestone 12,000 feet thick. The sedimentary 
mantl e is the Huronian of Logan, and the crystalline basement 
may conveniently be ca lled the pre-Huronian . The Huronian 
sediments are separated by an unconformity into a lower and 
an upper series. They have been folded into a synclinal trough 
extending from northwest to southeast. The sides of this 
trough dip 50 degrees or less and have been relatively displaced 
about 5,000 feet vertically by a great fault that extends along the 
bottom of the trough. Huronian and pre-Huronian -rocks are 
intruded by dykes and sill-like masses of diabase and the diabase 
is itself cut by small dykes of olivine-diabase, the youngest 
rock in the area. 
Undisturbed contacts between the base of the Huronian 
sediments and the underlying crystalline rocks do not occur 
within the area mapped, but are found just outside it, on some 
small islands in Lake Huron, 3 miles east of Thessalon. There, 
a basal conglomerate rests upon older, decomposed granite- 
gneiss and schist. Vaii Hise and Leith have described the con- 
tact of this conglomerate with the underlying rocks as follows: 
“Resting upon this complex” (granite, gneiss and included 
schist) “was found a great bowlder conglomerate which differs 
radically in character from the pseudo-conglomerates of the 
Laurentian. The pebbles and bowlders instead of being widely 
separated are packed closely together. Within a very small 
area, a square yard or square rod, may be found all varieties of the 
material occurring within the Basement Complex — that is, many 
phases of crystalline schist, gneiss, granite, and granite gneiss. 
On one of the islands in which the contact was seen the line of 
separation is perfectly sharp and irregular, bending at one place at 
