EARLY CAMBRIAN STRATIGRAPHY. 
113 
siderable portion of the Middle Cambrian seas were continuous 
between Nevada and British Columbia there can be little 
doubt. Apparently synchronous deposits occur in the Lower 
Cambrian of California, Nevada, Utah, British Columbia, and 
Alberta, though in the latter region limestone forming conditions 
obtained long before the actual disappearance of Olenellus, an 
event which is not believed by the writer to have been contem- 
poraneous with the close of the Lower Cambrian (see page 112). 
The Albertella fauna crosses the interval between Gordon 
creek, Montana, and Mount Bosworth, British Columbia, a 
distance of 300 miles, with only a modicum of change; and the 
Spence-Stephen-Titkana fauna of the Middle Cambrian, though 
characterizing different lithologic facies and occupying different 
positions with respect to the basal elastics in the areas where 
it has been discovered, still preserves its identity through the 
1,100 miles separating the Highland range of Nevada from the 
Mount Robson district of British Columbia and Alberta. This 
broad expanse of water was bounded on the east in northern 
Utah, southeastern Idaho, and central Montana by land areas 
whose submersion was gradual and upon whose Pre-Cambrian 
shores the sea matured from Lower to Middle Cambrian in 
age before it was able to mantle the Pre-Cambrian regolith. 
Without going into a discussion of the problems involved in 
the satisfactory solution for the continent as a whole of the 
true relations of the Lower to the Middle Cambrian, we may 
base our conclusion that the Cordilleran region offers a very 
close approximation to a gradual transition between these 
two units upon the following grounds: (a) the Mesonacidae, 
or Olenellus-\ike genera, are everywhere represented in the 
uppermost layers of the Lower Cambrian by Olenellus (s.s.) 
or the latest 1 if not the highest exponent of the group for which 
it has long been considered typical, the importance of the 
apparent annihilation of this genus over so large an area being 
tempered by the presence of a reeurrent(?) species in the Mount 
Stephen section of British Columbia; ( b ) in the Mount Robson 
region the upper portion of the rocks referred to the Lower 
Cambrian contains representatives of all of the genera ( Callavia , 
1 Walcott: Smithsonian Misc. Coll., vol. 53, No. 6, 1919, p. 248. 
