Ta.i'oiKDini of Lake Superior Befiion. — H'/hcJ/cI/. 335 
and simple entity which it appeal's to be, it is a complex sys- 
tem. The difficulties of the region are great, but sufficient 
has been learned to warrant some leading distinctions. The 
unique and definite conception which may be derived from 
the published descriptions of the Keweenawan gradually 
crumbles away when one goes into the field and carefully notes 
the facts. He first experiences a profound bewilderment, 
from which he emerges with sharpened eagerness to solve the 
conflict between his preconceived notions and the apparently 
anomalous facts. This pursuit, in the case of the Minnesota 
sui'vey, has been continued during several years. Occasion- 
ally new data have been discovered which have led to changes 
in interpretation, and later discoveries have necessitated still 
further improvements. No attempt has been made, however, 
until now to group the entire system in a consistent structural 
scheme. Even now the discussion is but partial and the 
classification provisional. It has yet to be rounded out with 
a fund of fact that cannot here be presented, and it may have 
to be still further changed, but as a classification it rests on a 
lai'ge amount of field work and of compnrative study of the 
published literature. 
In concluding this series of ])apers it is interesting to note 
how beautifully the grand succession of geologic events in 
America in Cambrian time compares with the succession in 
Europe. Dr. Hicks has recently remarked as follows:* 
There can be uo doubt that the genera OlenelluH, Paradoxidea and 
Olenus iu the areas where they have been found to succeed each other 
in conformable sediments do mark very definite periods in the world's 
history: but such arf)itrary lines are not natural, and we must expect 
to find from time to time that the limit assigned to a genus will have to 
be extended as new areas are being explored. Where a genus, which 
has been sufficiently ;il)undant to characterize a main zone, disappears 
suddenly, there is usually .some indication in the deposits of at least a 
slight physical change. At St David's this is particularly marked, for 
immediately below the lowest I'anulnxidcs horizon a fine conglomerate 
containing angular fragments of volcanic material occurs, and this I 
have taken as the boinidary line berween the Caerfai iOle iiellus hoiis) 
and the overlying Solva (/V/^^»/wV^/ and J'(ir<idu,rideN l)ed8). Again at 
the top of the Menevian, and .separating it from the overlying Lingula 
flags (Olenus beds), massive grits succeed black slates and in these grits 
again we meet with volcanic matei-ials. Were it not for these changes 
*Life Zones in Paknrzoic Rocks, Geologieal Magazine, dec. iv, vol. i, 
p. 404, 1894. 
