THE PRE-RICHMOND UNCONFORMITY 525 
those in the Mississippi Valley. In this western region the great mass 
of the Big Horn limestone corresponds essentially in age to the Kimms- 
wick limestone, but at the summit of the formation, in discontinuous, 
pocket-like areas, as if deposited unconformably upon the: eroded or 
weathered surface of the subjacent beds, are strata bearing a rich 
and typical Richmond fauna with Rhynchotrema capax. 
With this unconformity clearly established in the Mississippi 
Valley and in the Big Horn Mountains, and with the faunal condi- 
tions so nearly alike in the two regions, we seem to be justified in con- 
sidering the same unconformity as being present in all the intervening 
area. If this conclusion is well founded, there must have been in 
post-Trenton time a great expanse of dry land in the interior of the 
North American continent, reaching from the Cincinnati arch prob- 
ably to the Rocky Mountain region, which in Richmond time was 
resubmerged. 
Such a widespread emergence and resubmergence of the interior 
of the continent should constitute an important and clearly marked 
epoch in geologic history, perhaps of sufficient importance to rank as 
a major division in this history. As we now understand the facts, 
there seems to be no such conspicuous break between the recognized 
summit of the Ordovician and the base of the Silurian, as this pre- 
ceding the Richmond. A thorough and systematic investigation of 
the entire Richmond and Maquoketa fauna, together with a careful 
comparison with the earliest Silurian faunas is highly desirable. It 
is especially desirable that the earliest faunas of the Medina sand- 
stone of the east be better known, in order to determine whether there 
exists aay relationship between them and the Richmond faunas of 
the Mississippi Valley. In a certain way the Richmond is perhaps 
analogous with the Helderbergian which was formerly considered 
as the youngest Silurian, but is now almost universally considered as 
the oldest Devonian. It is certainly within the limits of possibility 
that, with a more complete knowledge of the facts, the Richmond may 
be transferred in a similar way, and be considered as the oldest 
Silurian, rather than as the youngest Ordovician. 
