Cambrian Rocks. 57 



It is probable, however, that only a few persons 

 would as yet agree with me in this classification, and 

 indeed, since the first publication, by Mr. Murchison, of 

 ' The Silurian System,' dedicated to Professor Sedgwick 

 in 1839, there has been, after a temporary lull, but 

 little unanimity among British geologists on a subject 

 about which European and American geologists care 

 but little, and which is to a great extent a matter of 

 local opinion. 



In 1841 and 1842 Sir Henry de la Beche and those 

 who worked with him, adopted the term Cambrian 

 for all the purple grits and slates of St. David's and the 

 Longmynd, then supposed to be unfossiliferous ; while 

 the name Silurian, nearly in the same sense as used by 

 Murchison, was employed for all the strata between the 

 uppermost beds of these rocks and the top of the Ludlow 

 series. When the Government Geological Survey 

 reached North Wales this classification continued for 

 a time unchallenged. Professor Sedgwick had pre- 

 viously called the equivalents of part of these strata in 

 the north of England the CUMBRIAN series, and at that 

 time he called the blue and grey slaty series of Wales 

 the CAMBRIAN series, on the assumption, then unques- 

 tioned, that they were all older than the recognised 

 Llandeilo flags of Murchison. But in the progress of 

 investigation by Sedgwick and many others, it ap- 

 peared that his original Cambrian, and Murchison's 

 original Lower Silurian strata, were in great part 

 equivalent, and the great Professor of Cambridge 

 naturally reclaimed all that part of his kingdom, the 

 boundaries of which had, for all Wales, not been clearly 

 defined when he first tried to subdue it. He therefore, 

 maintained, that the true Cambrian series included all 

 the strata from the base of the purple slates and grits 



