228 A. P. COLEMAN—CLASTIC HURONIAN ROCKS. 
that an immense lapse of time separates the conglomerate and the un- 
derlying Keewatin, long enough for a coarse-grained plutonic rock to 
solidify, probably at considerable depths, and then for the region to be 
so profoundly eroded as to provide pebbles of the plutonic rock ona 
seashore. It is probable that the Keewatin conglomerates which have 
been referred to represent an important interval of erosion, perhaps 
equivalent to the one shown to exist by Van Hise and others between 
the upper and lower Huronian in the states to the south.* 
Nevertheless the striking difference in the character of the rocks of the 
two series, wholly sandy sediments in the Couchiching, largely diabase 
and porphyry and the products of their alteration in the Keewatin, 
shows that conditions had greatly changed before the later series was 
formed. 
It must not be assumed, of course, that all of the eruptives found in 
the Keewatin were surface flows of the same age as the enclosing rocks. 
Many of them are probably of the nature of laccolitic sills like the 
trap sheets in the Animikie near Thunder bay. In fact, but for the un- 
doubted pyroclastic rocks among the sediments one might suspect that 
most of them were injected between the sedimentary beds, perhaps at a 
much later date, since no amygdaloidal varieties have been found. In 
a region where there has been so much folding and shearing it is rash to 
make positive statements on such matters, however. 
THICKNESS OF THE HURONIAN SERIES 
Lawson estimates the thickness of the Keewatin at about 5 miles, and 
of the Couchiching at about the same; { but W.H.C. Smith suggests for 
the latter that there may bea number of closely appressed folds not easily 
separated, so that the thickness of the Couchiching may be very much 
less, though still reaching 8,000 or 9,000 feet.§ 
Following Lawson’s estimate, the two series together sum up to 50,000 
or more feet in thickness, though it is probable that the lowest beds of 
the Couchiching have been dissolved by the molten Laurentian rocks 
beneath, since no basal conglomerate has been found; and also that 
there has been a considerable amount of compression during the squeez- 
ing undergone in the sharp synclinal folds, as proved by the flattening 
of soft pebbles in the conglomerates. 
The Couchiching, containing some little consolidated sandstones, can 
scarcely be included in the Laurentian; and as it forms transitions to 
* Journal of Geology, vol. i, No.2, p. 120. 
+ Geol. Survey of Canada, 1887, p. 55, F. 
{ Ibid., pp. 101 and 102, F. 
2 Ibid., 1890-91, pp. 54 and 55, G, 
