444 HISTORY OF GEOLOGY AND PALAEONTOLOGY. 



retained for the Lower Silurian of Murchison ; and the term 

 Silurian System be limited to the Wenlock and Ludlow 

 groups. Marcou justly pointed out that all the fossils ascribed 

 by Sedgwick and M'Coy as characteristic of Sedgwick's Upper 

 Cambrian (Bala) series were Lower Silurian fossils, whereas 

 distinctive fossil-types had been found by Emmons in the 

 " Taconic System," hence the latter term ought to be applied 

 generally to primordial rocks containing that fauna. But 

 Barrande had observed in 1851 in the British Survey 

 Collection a fossil Trilobite of primordial age, and Salter 

 afterwards discovered the localities in Wales whence certain 

 pre-Silurian fossils were derived. The " Lingula " flags and 

 shales of St. David's proved richly fossiliferous, and after these 

 had been described by Salter and Hicks (1868), there could 

 no longer be any question that there existed a distinctive fauna 

 in Sedgwick's original "Cambrian Series." 



It is largely due to Lyell's example that the name of "Cam- 

 brian " was retained in the text-books, at first usually as a 

 sub-division of the Silurian system, but finally as a system of 

 equal rank with the Silurian. 



The Cambridge School continued until recently to teach, in 

 accordance with Sedgwick's views, that the limit between the 

 Cambrian and Silurian systems was above the Bala beds. 

 Lyell, in his Elements of Geology r , limited the Cambrian system 

 to the lower and middle members of Sedgwick's system, begin- 

 ning with the Longmynd strata and ascending to the Tremadoc 

 skites; and in 1888, at the International Congress in London, 

 this limit was sanctioned and has since been almost universally 

 adopted. 



In the year 1879, Lapworth proposed the designation Ordo- 

 vician for the complex of strata which had been variously 

 termed Lower Silurian or Upper Cambrian. Lapworth's de- 

 tailed research and intimate knowledge of the group led him 

 to the opinion that it should be ranked as an independent 

 system, as it was distinguished from the rocks above and 

 below, not only by the occurrence of distinct fossil types but 

 likewise by the intercalation of lavas, tuffs, and ashes amidst 

 its sedimentary series. The Ordovician system has been sub- 

 divided by Lapworth upon palseontological grounds into a 

 Lower Ordovician or Arenig series, a Middle Ordovician or 

 Llandeilo series, and an Upper Ordovician or Bala series. 



A renewed investigation of Emmons' district in the United 



