CHAPTER V 



THE CAMBRIAN SYSTEM 



A. NOMENCLATURE AND CLASSIFICATION 



THE establishment of three separate systems among the Lower 

 Palaeozoic rocks was not accomplished without much controversy, 

 for in early days only two were recognised, and both were made 

 more comprehensive than they ought to have been. 



The well-known name Silurian (from the British tribe of Silures 

 who inhabited the western parts of Wales and its borders) was first 

 introduced by Sir Roderick Murchison in 1835 to designate the 

 series of rocks which he had studied in the land of the Silures 

 (Shropshire, Herefordshire, and South Wales) ; he divided this 

 system into an upper and a lower series. 1 a 



In the meantime, between the years 1831 and 1835, Professor 

 Sedgwick had worked out the complete succession of the Palaeozoic 

 rocks in North Wales, and in 1835 he proposed the name Cambrian 

 for this series, which he divided into three portions, calling them 

 Lower, Middle, and Upper Cambrian. For the beds which overlie 

 this series in the Berwyn Mountains he adopted Murchison's name 

 of Silurian. 2 



Sedgwick worked out the succession from below upwards, and 

 Murchison worked from above downwards, and for a long time 

 neither was aware of the fact that their systems overlapped each 

 other. Murchison supposed that Sedgwick's Cambrian lay entirely 

 below his Silurian, but when the fossils were collected and de- 

 scribed, it was found that the Upper Cambrian was equivalent to the 

 Lower Silurian, and this fact led to much subsequent controversy. 



As Murchison became the Director-General of the Geological 



Survey, his nomenclature was naturally adopted by the surveyors, 



and as they found that his Lower Silurian had no natural base, 



group after group of Sedgwick's Cambrian was gradually absorbed 



a For references see end of the chapter. 



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