296 
POPULAR SCIENCE REVIEW. 
Part II. 
Lower Palaeozoic. 
It is not intended to burden this paper with a long history 
of the various classifications which have been adopted for these 
rocks from time to time ; for classifications must necessarily 
vary with advancement of knowledge. All hard and fast lines, 
or the slavish adherence to any arrangement founded only 
in history and not in truth, are here, as in all other matters, 
barriers to progress. Numerous papers, however, on the ques- 
tion of history have of late been published, to which the reader 
may refer.* 
The nomenclature will of necessity in some cases have chiefly 
a local significance, but as many of the groups have been at 
present little more than locally examined, this is almost un- 
avoidable. Many of the names, however, are somewhat historical 
in their application, and have been so frequently used that they 
are now tolerably well known. The formations to be described 
are the Cambrian, Ordovian, and Silurian. 
1. Cambrian. 
Wherever these rocks are found in this country in contact 
with the Eozoic formations, they are unconformable to the latter. 
Fragments of the older rocks are also found in them in abun- 
dance, and evidence that they must have been built up chiefiy 
from the denudation of the Eozoic rocks is everywhere ap- 
parent. 
The name Cambrian, for these oldest Palaeozoic rocks, was 
first used by Prof. Sedgwick, and it was he who laid the founda- 
tion for all future discoveries in these rocks. It is impossible 
to speak too highly of the value of his researches amongst 
these at that time barren strata ; and if we cannot at present 
accept fully the divisions, or subdivisions, which he proposed, 
we reject them with much regret, and because their retention 
would greatly tend to retard the progress of science. 
This formation may with advantage be divided into two main 
portions, with several groups in each. 
Th eLoicer Cambrian has a natural base made up of massive con- 
glomerates and sandstones (see Sect. l,p. 308) ; its upper boundary 
is well defined by an important palaeontological break. It is 
* See Hunt, Canadian Naturalist, 1872, and Geol. Essays, 1878 ; Ethe- 
ridge, Quart. Joum. Geol. Soc. May, 1881 ; Marr, Geol. Mag. June 1881 ; 
Lapworth, Geol. Mag. June and July, 1881; and Hicks, Proceedings Geol. 
Assoc. 1873, Quart. Joum. Geol. Soc. 1875, and Geol. Mag. 1876. 
