100 THE CAMBRIAN SYSTEM OF STRATA. 
Cambrian upward into the higher fossiliferous rocks 
closely related to the barren Cambrian strata first studied. 
Still Sedgwick paid little attention to the paleontology 
in drawing his conclusions ; he collected many fossils 
and laid them aside for future study, but he classified 
the rock on the general principles of comparative strati- 
graphy, somewhat independently of the paleontology. 
Thus he continued to work upward until he claimed as 
Cambrian strata not only the lowest ‘‘ barren shales and 
grits’? which Murchison had allowed him, but the higher 
strata up to the very base of the Upper Silurian. Thus 
Murchison’s ‘‘ Silurian’? and Sedgwick’s ‘‘ Cambrian ”’ 
overlapped each other in a confusion which has never 
since been resolved.* 
The title ‘‘ Primordial Zone,’’ proposed by Barrande 
for certain of the lower strata did not contribute any 
relief to the situation. 
A fair compromise has, however, been provisionally 
effected by some of the eminent British geologists by 
calling the entire mass of Sub-Devonian strata Silurian, 
and then dividing them into three members. ‘The lowest 
member is called the Cambrian or Primordial Silurian, 
and is itself subdivided into two series, the lower com- 
prising the Harlech, Longmynd and Menevian Groups, 
and the higher the Lingula Flags and, according to some 
authorities, the Tremadoc Slates also. The second mem- 
ber is called the Lower Silurian. Beginning where the 
Cambrian Silurian leaves off, it extends through the 
Lower Llandovery. The third member, called the Upper 
Silurian, beginning with the Upper Llandovery, extends 
to the Devonian. 
* For a more complete presentation of this topic see an article pub- 
lished by Professor James D. Dana since the reading of the present pa- 
per, entitled ‘‘Sedgwick and Murchison ; Cambrian and Silurian,” in 
the American Journal of Science, March, 1890. 
5O 
