_ Parr IL Secr.i.§ 2.) CAMBRIAN. 658 
system. He therefore included as Cambrian only the barren grits 
and slates of the Longmynd, Harlech, and Llanberis. Sedgwick, on the 
other hand, insisted on carrying the line up to the base of the Upper 
Silurian rocks. He thus left these rocks as alone constituting the 
Silurian system, and massed all the Lower Silurian in. his Cambrian 
system. Murchison worked out the stratigraphical order of succession 
from above, and chiefly by help of organic remains. He advanced fron. 
where the superposition of the rocks is clear and undoubted, and for the 
first time in the history of geology ascertained that the “ transition- 
rocks” of the older geologists could be arranged into zones by means of 
characteristic fossils as satisfactorily as the Secondary formations had 
been classified in a similar manner by William Smith. Year by year, as 
he found his Silurian types of hfe descend farther and farther into lower 
deposits, he pushed backward the limits of his Silurian system. In this 
he was supported by the general consent of geologists and paleeontologists 
all over the world. Sedgwick, on the other hand, attacked the problem 
rather from the point of stratigraphy and geological structure. ‘Though 
he had collected fossils from many of the rocks of which he had made out 
the true order of succession in North Wales, he allowed them to lie for 
years unexamined. Meanwhile Murchison had studied the prolongations 
of some of the same rocks into South Wales, and had obtained from them 
the copious suite of organic remains which characterized his Lower 
Silurian formations. Similar fossils were found abundantly on the ~ 
continent of Europe and in America. Naturally the classification pro- 
posed by Murchison was generally adopted. As he included in his 
Silurian system the oldest rocks containing a distinctive fauna of trilo- 
bites and brachiopods, the earliest fossiliferous rocks were everywhere 
classed as Silurian. The name Cambrian was regarded by geologists of 
other countries as the designation of a British series of more ancient 
deposits not characterized by peculiar organic remains, and therefore 
not capable of being elsewhere satisfactorily recognized. Barrande, in- 
vestigating the most ancient fossiliferous rocks of Bohemia, distinguished 
by the name of the “Primordial Zone” a group of strata underlying 
the Lower Silurian rocks, and containing a peculiar and characteristic 
suite of trilobites. He classed it, however, with the Silurian system, 
and Murchison adopted the term, grouping under it the lowest dark 
slates which in Wales and the border English counties contained some 
of the same early forms of life. 
More recent investigations, however, first by the late Mr. Salter and 
Dr. Hicks, and subsequently by the latter observer, brought to light, 
from the so-called primordial rocks of Wales, a much more numerous 
fauna than they were supposed to possess, and one in large measure 
distinct from that in the undoubted Lower Silurian rocks. Thus the 
question of the proper base of the Silurian system was re-opened, and 
the claims of the Cambrian system to a great upward extension were 
more forcibly urged than ever. But these claims could now be based on 
paleontological evidence such as had never before been produced. Ac- 
cordingly there has arisen a general desire among the geologists of 
Britain to revise the nomenclature of the older rocks. Though as yet a 
common accord of opinion has not been reached, there seems a strong 
probability that ultimately the boundary line between the Cambrian 
and Silurian systems will be drawn above the primordial zone along the 
