196 CHAREES, DWALCOLT 
central land-area the advancing seas forced the faunas to shift their 
habitat and either to adjust themselves to the new conditions or to 
perish. Local isolation for long periods led to the development of new 
forms, and these, when the barriers were removed, contested and com- 
peted for their position and life with other faunas until, by a process of 
elimination of those least fit to survive, there was hastened the devel- 
opment of a large and varied fauna. With the close of Middle Cam- 
brian time more stable conditions returned, and the era of rapid 
evolution was checked until the impulse of new conditions of environ- 
ment and an accumulated tendency to change resulted in the great 
evolution of life in the lower Ordovician. 
LIFE AT THE BEGINNING OF KNOWN CAMBRIAN TIME 
The traces of pre-Cambrian life, though very meager, are sufficient 
to indicate that the development of life was well advanced long before 
Cambrian time began. The characteristic fossil of the known pre- 
Cambrian fauna is Beltina danai,! a crustacean probably more highly 
organized than the trilobite. The associated annelid trails indicate 
that this phase of the fauna was also strongly developed.  Strati- 
graphically, this fragment of what must have been a large fauna occurs 
over 9,000 feet beneath an unconformity at the base of the upper por- 
tion of the Lower Cambrian in northern Montana.” Thisfact indicates 
that it is practically hopeless to search for the first forms of life—those 
that could leave a trace of their existence—in strata now referred to 
the Cambrian or early Paleozoic. With this thought in mind we shall 
consider what is known of the life of early Lower Cambrian (Georgian) 
time. 
The oldest known Cambrian fossils are found deep down in the 
Lower Cambrian strata of southwestern Nevada and the adjoining 
Inyo County area of eastern California. In sections 120 miles apart 
the Lower Cambrian has a thickness of over 5,000 feet, with a great 
limestone forming the upper 700 to 2,000 feet. Below this limestone 
calcareous strata occur, but the predominating rocks are sandstones, 
and arenaceous, siliceous, and calcareous shales. In the lower 400 
t Bull. Geol. Soc. Amer., Vol. X, 1899, pp. 238, 239. 
2C. D. Walcott, Observations of 1908. 
