260 The Founders of Geology LECT. 



separated from everything younger than themselves by a 

 strong unconformability. The Carboniferous and Old 

 Eed Sandstone strata were found to lie on the upturned 

 edges of the older masses, and it was impossible to say how 

 many intervening formations were missing. 



Murchison's researches brought to light the actual 

 transition from the base of the Old Eed Sandstone into an 

 older series of fossiliferous formations underneath. There 

 could, therefore, be no doubt that part at least of his 

 Silurian system was younger than Sedgwick's series in 

 North Wales. And as he found what appeared to be 

 older strata emerging from underneath his system, and 

 seeming to stretch indefinitely into the heart of Wales, he 

 naturally believed these strata to be part of his friend's 

 domain, and at first left them alone. Such, too, was Sedg- 

 wick's original impression. The two fellow -workers had 

 not drawn a definite boundary between their respective 

 territories, but they agreed that the Silurian series was 

 less ancient than the rocks of North Wales. 



As a distinct name had been given to the younger 

 series, Murchison urged his associate to choose an ap- 

 propriate designation for the older, and in the summer of 

 1835 the term "Cambrian" was selected. 1 By this time 

 Murchison had learnt that no hard and fast line was to 

 to be drawn between the bottom of the Silurian and the 

 top of the Cambrian series. " In South Wales he had 

 traced many distinct passages from the lowest member of 

 the ' Silurian system ' into the underlying slaty rocks now 



1 Brit. Assoc. August 1835, Phil. Mag. vol. vii. (December 1835), p. 

 483, "On the Silurian and Cambrian Systems " by A. Sedgwick and K. 

 I. Murchison. 



