EARLY CAMBRIAN STRATIGRAPHY. 
115 
in the contemporaneous seas of Nevada and California held 
back until the very last. Farther north, in British Columbia, 
however, the genus ranges sparingly through the upper 500 feet 
of a similar clastic series and then occurs in great profusion in 
the lower 20 feet of an overlying transitional formation. 
Here the true Lower-Middle Cambrian boundary from a 
diastrophic standpoint would appear to lie just above this 
clastic series, but the profusion of Lower Cambrian forms in 
the overlying 20 feet 1 suggests the drawing of the boundary 
above that stratum, and the correctness of this interpretation 
appears to be attested by the occurrence, in the immediately 
overlying beds, of the Middle Cambrian Albertella fauna with 
its host of new types (See pages 118-119). 
A similar delineation of the boundary between the Lower 
and Middle Cambrian has been made in Big Cottonwood 
canyon, Utah, in the belief that where so distinctive a Lower 
Cambrian form as Olenellus occurs just above a quartzitic 
series, even in transitional beds, and is neither associated with 
nor preceded by forms characteristic of the Middle Cambrian, 
w r e are not doing violence to the principles to which we subscribe 
(pages 119-120) when we suggest that the divisional boundary 
between the Lower and Middle Cambrian be drawn above the 
horizon characterized by that genus. 
In our opinion the sudden and widespread introduction of so 
characteristic a biota as the Albertella fauna was an event of 
far more importance than the ultimate extinction of the Mes- 
onacidte, and the Lower-Middle Cambrian boundary has been 
drawn below that horizon. When the absolute disappearance 
of the preceding biota or of any one type is not considered a 
necessary corollary to the inauguration of a new period, incon- 
sistencies wffiich appeared to be difficult of explanation no 
longer vex us but give way to new problems, and the delimitation 
of the boundaries between the stratigraphic units becomes one of 
increasing complexity because of the substitution of the proper 
valuation of a debatable series of diastrophic and organic 
phenomena for a simple yes or no. 
1 Walcott: Smithsonian Misc. Coll. vol. 53, No. 5, 1908, pp. 214-215. 
