1895.] The First Fauna of the Earth. 887 
that it was at last decided that these “ Georgia slates” were 
older than the Potsdam, but not as old as the Braintree, Mass., 
beds, in which Paradoxides had been found. Prof. Hall had 
established the genus Olenellus to include the Vermont trilo- 
bites, and the idea prevailed that this genus succeeded Para- 
doxides in time. It was in 1868 that the first reference was 
made of the Potsdam rocks to the top of the Primordial period, 
instead of to the base of the Silurian where they had pre- 
viously been placed. So that at this time the Braintree beds 
were supposed to contain the oldest fossils on the globe. 
Meanwhile, geologists had been studying the fauna in rocks 
occurring about St. John, New Brunswick. Noting the re- 
semblance the trilobites there bore to those from Braintree, 
they concluded the two deposits were of the same age. In 
Canada, Logan, in 1864, taking cognizance of all the discover- 
ies in New York, Vermont, Massachusetts, New Brunswick 
and Newfoundland, published a scheme of classification which, 
for twenty-four years, perpetuated an error. This scheme in 
its lower portion is as follows: 
(3) Upper Potsdam, including the rocks of the upper Missis- 
sippi Valley, northern New York and adjacent parts of Canada. 
(2) Lower Potsdam, including the rocks of Georgia, Vermont, 
and some of Newfoundland. 
(1) St. John Group, including the rocks at Braintree, Mass., 
St. John, New Brunswick, and St. John’s, Newfoundland. 
This view of the succession of the oldest fossil-bearings rocks 
of North America was held until 1888, except that the three 
divisions were called respectively, (3) Upper Cambrian, (2) 
Middle Cambrian, and (1) Lower Cambrian. Of these divi- 
sions the Upper was also called the Dikellocephalus zone, the 
Middle the Olenellus zone, and the Lower the Paradozides zone, 
from the three genera of trilobites confined to the rocks of each 
terrane. 
(To be continued.) 
