No. 381.] A HALF-CENTURY OF EVOLUTION. 635 
Taking as an example of the nature of the Algonkian 
changes one region alone, the Lake Superior region, where 
the stratigraphical record is more complete, we have : — 
I. The Lower Huronian schists, limestone, quartzites, con- 
glomerates, etc., with their eruptives, closely folded and attain- 
ing a maximum thickness of probably over 5000 feet. 
2. The Upper Huronian, unconformable to the Lower, a 
series of more gently folded schists, slates, quartzites, con- 
glomerates, interbedded and cut by trap, with a maximum 
thickness of 12,000 feet. In the Animikie quartzites of this 
age have, according to Selwyn, been detected a track of organic 
origin, and in the Minnesota quartzites, Lingula-like forms, as 
well as obscure “ trilobitic-looking ” impressions; while carbo- 
naceous shales are abundant. 
3. Between these Huronian rocks and the true Cambrian 
series are interpolated the Keweenawan elastic rocks, with a 
maximum thickness of 50,000 feet. Though these beds are by 
some high authorities referred to the Cambrian, the fact remains 
that this series, whether Cambrian or Algonkian, is unconform- 
able to the Huronian, and composed of fragmental rocks, the 
upper division being 15,000 feet thick, and consisting wholly 
of detrital material largely derived from the volcanoes of the 
same series. Between each series is an unconformity repre- 
senting an interval of time long enough for the land to have 
been raised above the seas, for the rocks to have been folded 
and to have lost by erosion thousands of feet, and for the land 
to have sunk below the surface of the ocean. 
Again, between the Precambrian and Cambrian there was, 
according to Walcott, a great uplift and folding of rock, 
succeeded by long-sustained erosion, over all the continental 
area. It was not, however, he states, “as profound as the one 
preceding: Algonkian time, as is proved by the more highly 
contorted and disturbed Archzan rocks beneath the relatively 
less disturbed Algonkian series.” ! 
The evidence of the existence of life forms in the Huronian 
and Keweenawan times is indicated by the presence of thick 
1 The North American Continent during Cambrian Time. Zwelfth Ann. Rep. 
U.S. Geological Survey, p. 544. 
