218 
REPORT OP THE AMERICAN COMMITTEE. 
Phase II., 1844. 
a. Black slate. Fossiliferous, 
b. Taconic slate, " 
Sparry limestone, . 
Magnesian slates, . 
Stockbridge limestone, . 
Granular quartz, . 
I. Cambrian. 
Mostly Hudson slate. 
II. Lower Silurian limestone. 
III. Hudson slates. 
II. Lower Silurian limestone. 
I. Cambrian. 
Phase IIL, 1855. 
I, 
, Upper Taconic. 
2. Black slate, . 
1. Taconic slate, 
. 
I. 
III. 
Cambrian. 
Mostly Hudson 
slate. 
II. 
, Lower Taconic. 
3. Magnesian slate, . 
2. Stockbridge limestone 
limestone, . 
1. Granular quartz, . 
and Sparry 
in. 
II. 
I. 
Hudson slate. 
Lower Silurian 
Cambrian. 
limestone. 
Classification of North American Cambrian Rocks. 
In the classification of the fossiliferous sedimentary rocks of 
all countries it becomes more and more evident that the great 
systems — Cambrian, Silurian, Devonian, etc. — must rest on the 
broad zoologic characters of their included faunas and not on 
stratigraphic breaks between the systems, and that geologists 
will need to recognize the fact so well stated by Lapworth, that 
" we have no reliable chronological scale in geology but such as 
is afforded by the relative magnitude of zoological change — in 
other words, that the geological duration and importance of any 
system is in strict proportion to the comparative magnitude and 
distinctness of its collective fauna."* In pursuance of the above 
principle I have separated the Cambrian System in North 
America from the Lower Silurian. In the magnitude of sedi- 
mentation and extent of the fauna it ranks with the other great 
geologic systems, and we cannot unite it with the Lower Silurian 
except from reasons that, if followed out, will unite all the sys- 
tems from the Cambrian to the Quaternary. 
In arranging the different strata composing the Cambrian 
System three primary divisions are distinguished by the pre- 
dominance in each of a fauna that, in assemblage of genera and 
species, may be separated from others whenever two or more of 
them occur in the same stratigraphic section. This extends to 
the identification of the relative geologic horizon by the fauna 
Geol. Mag., vol. vi., p. 3, 1879. 
