1907-8.] THE GEOLOGY OF THE LAKE WENDIGOKAN REGION. 347 
give rise to some of the arkose. The loose materials on the land surface, 
covered and sorted by the sea, would of course be supplemented by those 
brought down by the streams, but it is probable that during a time of sub- 
mergence the streams would be unable to transport much coarse material, 
and that so long as the lands remained comparatively low, the coarser 
stuff would be left on the surface until such time as the land should begin 
to emerge and the streams be rejuvenated, when it would be carried down 
and deposited in the retreating sea. I think it is quite probable that 
much of the material which formed the arkose was deposited in the re- 
treating sea and thus suffered little water action, because if it had been 
deposited at the same time as the conglomerate it must certainly have 
been more water worn than it is, as many of the hard conglomerate pebbles 
are well rounded. It is quite probable that part of the arkose was formed 
by the crushing and weathering of granite and quartz-diorites, and that 
it is a land formation of fragmental materials washed from the high to 
the low ground after the emergence of the land, but the close relation 
to the conglomerate seems to show that it was not all formed in this way. 
Whatever the origin, it is significant that the formation lies to the south 
of the great ridge of conglomerate, where the sea covered the land, and 
Seems to be found only where that rock is well developed. The great 
band of coarse conglomerate stretching all along the northern side of the 
map and much farther east shows that the coast line was for a considerable 
period near this band. 
The arkose appears to be the last of the Lower Huronian series and 
the latest consolidated clastic formation of this region. Though this 
and the earlier rocks have since their origin been subject to great erosive 
action by which large masses, many hundreds of feet, have been removed 
and been subject to other great changes through metamorphic agents, 
there is no direct evidence that they have since Huronian times been 
covered by the sea or any other extensive body of water for a great pe- 
riod of time. It is possible that the later eruptive rocks were intruded 
into sedimentary rocks which were post-Lower Huronian in age, and 
which have since been removed by erosion, but there is no proof that 
this occurred. Probably during the Keweenawan period a large mass of 
diabase was intruded into and through the rocks of the region west of 
Lake Wendigokan and this with similar intrusions in the surrounding dis- 
tricts exerted considerable metamorphic action on the older rocks. 
Between the Keweenawan and Pleistocene none of the great periods 
are represented by rock systems, but the Pleistocene is well marked by 
sand plains and various kinds of drift deposits. All the ages have left 
no trace of life in the rocks of this region unless it be in the Carbonaceous 
