

170 J. D. Dana — Sedgwick and Murchison. 



tions from the top of the Berwyns to Bala. Murchison 

 concluded, after his brief examination, and told Sedgwick, that 

 the Bala group could not be brought within the limits of his 

 system. He says: "I believed it to plunge under the true 

 Llandeilo flags with Asaphus Buchii which I had recognized 

 on the east flank of that chain." " Not seeing, on that hurried 

 visit, any of the characteristic Llandeilo Trilobites in the Bala 

 limestone, I did not then identify that rock with the Llandeilo 

 flags, as has since been done by the Government surveyors."* 



In 1835, the terms "Silurian" and "Cambrian" first appear 

 in geological literature. Murchison named his system the 

 u Silurian " in an article in the Philosophical Magazine for 

 July of that year, and at the same time defined the two grand 

 subdivisions of the system : I, the Upper Silurian, or the 

 Ludlow and W enlock beds ; and II, the Lower Silurian, or 

 the Caradoc and Llandeilo beds.f 



During the next month, August, the fourth meeting of the 

 British Association was held at Edinburgh, and in the report of 

 the meeting,;}: the two terms, Silurian and CambrioM, are united 

 in the title of a communication " by Prof essor Sedgwick and 

 B,. I. Murchison," the title reading, " On the Silurian and 

 Cambrian systems, exhibiting the order in which the older 

 sedimentary strata succeed each other in England and Wales." 

 Murchison, after explaining his several subdivisions, said that 

 "in South Wales" he had "traced many distinct passages 

 from the lowest member of the ; Silurian system ' into the 

 underlying slaty rocks now named by Professor Sedgwick, the 

 Upper Cambrian. " Sedgwick spoke of his " Upper Cam- 

 brian group " as including the greater part of the chain of the 

 Berwyns, where he said, " it is connected with the Llandeilo 

 flags of the Silurian and expanded through a considerable part 

 of South Wales ;" the "Middle Cambrian group)" as "com- 

 prising the higher mountains of Caernarvonshire and Merion- 

 ethshire ;" the "Lower Cambrian group" as occupying the 

 southwest coast of Caernarvonshire, and consisting of chlorite 

 and mica schists, and some serpentine and granular limestone ; 

 and finally, he " explained the mode of connecting Mr. Mur- 

 chison's researches with his own so as to form one general 

 system." 



Thus, in four years Murchison had developed the true sys- 

 tem in the rocks he was studying ; and Sedgwick likewise had 

 reached what appeared to be a natural grouping of the rocks 

 of his complicated area. Further, in a united paper, or papers 

 presented together, they had announced the names Silurian 

 and Cambrian, and expressed their mutual satisfaction with the 



* Murchison, Q. J. G. Soc, viii, 115. f Phil. Mag., vii, 46, July, 1835. 



\ Brit. Assoc, v, August, 1835. 



