[COLEMAN | CLASSIFICATION OF THE ARCH ASAN 137 
he doubted whether the western rocks were really Huronian, and 
inclined to make them older than the typical Huronian. 
The most important discovery of Lawson was that the Laurentian 
was not the oldest rock of the region, since it has an eruptive contact 
with the Coutchiching and Keewatin, which must, therefore, have been 
solid rock before the gneiss of the Laurentian had cooled and solidi- 
fied. This discovery had far-reaching consequences, for if any part of 
the Keewatin was of Huronian age the generally accepted relation- 
ship of Huronian and Laurentian must be reversed. 
SUBDIVISIONS OF THE HURONIAN. 
Within the past few years evidence has accumulated showing 
that the Huronian as described and mapped by Logan and his suc- 
cessors in Ontario at least, must be subdivided into two formations, 
an upper and a lower, separated by a very extensive unconformity. 
The lower division corresponds in the main with Lawson’s Keewatin 
and Coutchiching; while the upper one includes most of the Huronian 
as found on the north shore of Lake Huron. 
The need for this division has been brought out by the tracing of 
the iron range rocks and of basal conglomerates containing pebbles 
of these rocks from the province of Quebec near Lake Temiscaming 
almost to the boundary of Manitoba on the Lake of the Woods, a 
distance of nearly 800 miles. As shown by the work of the Bureau 
of Mines of Ontario, every large area of rocks mapped as Huronian 
between these limits contains long belts or ridges of jasper, chert or 
granular silica interbanded with iron ore, and not far off thick bands 
of schist conglomerate or graywacke conglomerate containing pebbles 
of the banded silica.1 
The interval between the upper and lower formations was long 
enough to provide for very extensive erosion of hard crystalline rocks 
over an area of at least 800 by 100 miles, and the building up of 
hundreds and often thousands of feet of conglomerate, containing 
well rounded pebbles and boulders of very heterogeneous rocks, such 
as granite, quartz porphyry, various greenstones and schists, and the 
characteristic rocks of the iron range. There are few unconform- 
ities in Paleozoic or later times which can be compared with it 
for magnitude, though Lawson’s Eparchean interval, the break 
between the Huronian and the Cambrian, is no doubt much greater. 
The thick slate conglomerates of the Huronian north of Lake 
Huron with their numerous pebbles of jasper and chert evidently 


? Ont. Bur. Mines, 1901, Vol. 10, pp. 200-204. 
