434 HISTORY OF GEOLOGY AND PAL/EONTOLOGY. 



proposed by Sedgwick in 1836 for these older members, and 

 this term was accepted by Murchison. 



In the year 1839 Murchison published his great work The 

 Silurian System^ wherein the results of his researches 

 extending over six years were admirably elucidated. After a 

 short statement regarding the younger geological formations, 

 and a more detailed account of the English Carboniferous 

 formation, the Mountain Limestone, and the Old Red 

 Sandstone, Murchison passes to the special description of the 

 Silurian system in South Wales and the adjoining counties of 

 England. With great accuracy he depicts the stratigraphical 

 relations, the lithological characters of the rocks, the contents 

 of fossils and minerals, and the occurrences of volcanic rocks. 

 A special palaeontological part with twenty-seven quarto plates 

 is devoted to the description of the characteristic fossils by 

 L. Agassiz, Sowerby, and Lonsdale. Numerous coloured 

 sections help to demonstrate the tectonic relations of the 

 district. 



Murchison distinguishes three chief divisions in the Silurian 

 system 



Upper Silurian, comprising the Ludlow Rocks and 



Wenlock Limestone. 

 Lower Silurian, comprising the Caradoc Sandstone and 



Llandeilo Flags. 

 Cambrian. 



Murchison found it impossible at the time to fix a definite 

 palaeontological horizon as the lower limit of the Silurian 

 system, and Sedgwick also could not assign any palaeontological 

 or other feature which would determine the upper limit of the 

 Cambrian series. Nevertheless, the recognition of the Silurian 

 and Cambrian systems was one of the most important 

 advances that have been made in stratigraphy. 



There still remained, however, a thick group of strata in the 

 Wernerian "Transitional Series" which could not be allotted 

 to either of the newly-defined systems. De la Beche had 

 worked with unrelaxing energy for several years at the 

 geological investigation of Devonshire and Cornwall, and in 

 1839 had reproduced his results on an excellent geological 

 map of this district. He had separated a series of plant- 

 bearing shales from the true greywacke strata (Killas 

 Greywacke) and applied to them the name Carbonaceous 

 series (now "Culm Shales"), but he thought the latter was in 



