686 Henry Shaler Williams. 



"sufficient to occasion erosion in the southern area of the 

 Mississippian sea" (Scope of Paleontology, 1892). 



As early as 1894 Williams began to point out the neces- 

 sity of a dual nomenclature in geological classification. 

 He then clearly showed that geologic formations have (1) 

 a local and definite lithologic value in a sequence of strata, 

 which in a wider distribution may become more and more 

 indefinite lithologically ; and (2) a variable time value in 

 the general history of the earth and in the evolution of its 

 organisms. Formations therefore have two values, and 

 we should not confuse them in our geologic classification. 

 In other words, there should be two sets of geologic 

 terms, one expressive of sediments, and another of time. 



In his Correlation Papers of the Devonian and Carbon- 

 iferous (1891), Professor Williams found it impossible 

 to give "a thorough paleontologic definition of the sys- 

 tems and series under consideration. The result has 

 demonstrated that the facts are not yet accumulated to 

 make this possible. ' ' Later on we find that he helped to 

 delimit the upper boundary of the Devonian, drawing the 

 line successfully between the Chemung and Catskill 

 formations on the one side, and the Waverlian on the 

 other. 



In regard to the lower boundary of the Devonian, how- 

 ever, he was not so successful. At first he accepted the 

 prevalent view that the Oriskany forms the base of the 

 Devonian. This view was challenged in 1889 by J. M. 

 Clarke, who referred all of the Lower Helderberg to 

 the Devonian, and thus closed the Silurian with the 

 Waterlime or Bertie formation. This conclusion finally 

 brought forth Williams's paper The Silurian-Devonian 

 Boundary in North America, The Chapman Sandstone 

 Fauna (1900). Here he wrongly concludes that "The 

 Chapman fauna must be regarded as the equivalent of 

 the topmost fauna of the typical Welsh Silurian system 

 (= Upper Ludlow, Tilestone, Downton and Ledbury 

 formations) . . . and of the uppermost Arisaig 

 fauna of Nova Scotia." "This places the Silurian- 

 Devonian boundary for North America at the place 

 where it was determined by De Verneuil in 1847. ' ' On 

 the other hand, he is near the truth when he states that 

 the Chapman fauna "is equivalent to the Lower Oriskany 

 fauna" of New York. In a later paper, however, he cor- 

 rectly says that the Chapman fauna ' ' seems to be strictly 



