MARYLAND GEOLOGICAL SURVEY 25 



HISTOKICAL EEVIEW 



It will be remembered that previous to 1830 geologists grouped into a 

 single large and indefinite " Transition Series " all of the sedimentary 

 and interbedded volcanic rocks of Great Britain older than the Car- 

 boniferous. Immediately underlying the Carboniferous was the great 

 mass of red sandstones and marls first designated the Old Eed sandstone 

 and later determined as of Devonian age. In 1831, Sir Eoderick I. 

 Murchison and Professor Adam Sedgwick attacked the problem of the 

 division of the remaining underlying strata, confining their studies to 

 the rocks of western England and eastern and southeastern Wales. 

 Murchison undertook this study under favorable circumstances, as he 

 began his researches at the upper end of the series where fossils abound 

 and the structure is simple. He found that the different members of the 

 upper part of this great series could be recognized by the fossils as easily 

 as more recent strata, and he continued to discover fossiliferous zones 

 lower and lower in the series. He first designated these strata as the 

 "fossiliferous graywacke series," but in 1835 he changed this to the 

 " Silurian System," named after the Silures, a tribe of ancient Britons. 

 Sedgwick, on the other hand, attempted the division of the transition 

 series in the Snowdon district of Wales where the complicated structure 

 and highly altered nature of the rocks, and consequent scarcity of fossils, 

 made the problem an extremely difficult one. In 1835 he proposed the 

 name " Cambrian Series " for the lower, older member, taking the name 

 from Cambria, the Roman name for northern Wales. 



Murchison divided his Silurian system into an upper and lower portion 

 which were separated from each other, as pointed out by Sedgwick, by an 

 angular unconformity marking the boundary between the Caradoc and the 

 Llandovery groups. The lower limit of his Silurian was not defined, but 

 he finally included all of the fossiliferous strata of Sedgwick's Cambrian. 



Murchison. in his volume on the Silurian system published in 1839, 

 recognized the Cambrian series, and up to this point the two workers 

 agreed. However, in 1842, in his presidential address to the Geological 

 Society of London, he stated that the Cambrian fossils did not differ from 



