GENERAL MEETING. 99 
Nevada the section passes through limestones marked by the presence 
of a typical Potsdam fauna and on up to one that has the general 
facies of that of the Trenton Lower Silurian fauna. Midway of 
these passage beds occur layers of rock that carry representatives 
of both the Cambrian and Silurian faunas. Above this band the 
Cambrian fauna gradually disappears, and below it soon predomi- 
nates to the exclusion of the Silurian types. In this section we have 
an illustration of the gradual extinction of an older fauna as a new 
one is introduced, the sedimentation continuing and no physical dis- 
turbance occurring to change the conditions necessary for the pres- 
ence of animal life. It is the ideal section uniting the faunas of two 
periods, and if we had the blanks filled in between all the groups, 
as the blank between the Potsdam and Chazy in New York is filled 
in by the Nevada section, the Paleozoic would be a record of con- 
tinuous connected organic life from the base of the Cambrian to 
the summit of the Permian. 
It is convenient for stratigraphic geologic work to separate the 
Paleozoic series into subdivisions, and, as this is almost necessarily 
done on paleontologic evidence, I would separate the Cambrian as 
one characterized by what Barrande has named the first fauna.* 
Applying this to the Nevada section already mentioned, the line 
between the Cambrian and Silurian would be drawn where the 
types of the second fauna begin to predominate. With this defini- 
tion of the Cambrian system, the strata referred to it in the United 
States and Canada will be briefly noticed. 
In the Grand Cafion of the Colorado the top of the Cambrian is 
the Tonto formation, a series of sandy calcareous strata 1,000 feet 
in thickness. The contained fauna is closely allied to that of the 
Potsdam sandstone and continues up to the summit of the forma- 
tion, the overlying Devonian rocks resting directly above strata 
containing Lingulepis, Iphidea, Conocephalites, Dicellocephalus, etc. 
The Tonto rests uncomfortably on strata that were extensively 
eroded prior its deposition. This lower series comprises over 11,000 
feet of unmetamorphosed shales, limestones, and sandstones, with 
1,000 feet of interbedded lavas. It forms the Grand Cajion and 
Chu-ar’ groups of Powell and is characterized by the presence of a 
few fossils that enable us to refer it to the Cambrian but not to de- 
fine its stratigraphic horizon. That is done on the evidence of the 
position it occupies with reference to the Tonto. 
* The paleontologic evidence and discussion will appear in a future paper. 
