ART. 21 ORDOVICIAN TRILOBITES ULRICH 63 



which I have used the British term Silurian since 1910. But it does 

 not agree in either its base or its top with Lapworth's Silurian system. 

 As defined by him the Silurian system in Britain comprises the 

 " strata comprehended between the base of the Old Red sandstone 

 (Devonian system) and that of the Lower Llandovery ^'' which dif- 

 fers considerably from the " Ontario group " of Emmons, the origi- 

 nal limits of which were retained without modification when most 

 of us discarded its American designation and adopted Silurian in 

 its place. Lapworth's definition includes beds corresponding to our 

 Helderbergian series, which all American geologists now refer to 

 the base of the Devonian ; and it excludes and refers to the Ordi- 

 vician all beds in Great Britain that are older than the base of our 

 Clinton, whereas in American practice without exception the 

 Silurian includes the whole or at least the ui:)per half of the under- 

 lying Medinan series. 



The differences between the British and American practice in 

 drawing the top and bottom limits of the Silurian arise in part 

 from differences in methods. The former inclines, at least in these 

 cases, to the practice of beginning a system at a stratigraphic break 

 that immediately underlies the first well-established change from 

 the dominant character of the fauna of the preceding period to 

 that of the succeeding period. The latter inclines rather to the 

 practice of begining the new system with the first diastrophically 

 well marked introduction of the new fauna. But, for obvious 

 reasons — particularly as regards difference or likeness in source of 

 the compared faunas and the greatly varying dates at which marine 

 deposition of a given period began or ceased in the numerous epi- 

 continental basins — the degree of difference in the general aspect of 

 the faunas in beds that are contiguous yet of different periods 

 depends very largely on only locally operating factors. Thus, if the 

 faunas of such adjoining beds invaded from the same oceanic realm 

 the younger of the two is likely to comprise a strongly dominant 

 part that is made up of direct descendents of the older fauna; if 

 they invaded from different realms then the difference between the 

 two is much greater and often complete. 



Even in different parts of the same continental province the firet 

 deposit of a given period may be shown by its fossils to be either 

 much younger or older than is the first of the period in other 

 parts. In some places only the lower series of a system may be 

 represented, in another only its middle, and this may then be fol- 

 lowed by the closing stage. Again, in some places only the closing 

 series is represented whereas in other places beds of the closing series 

 are in contact with deposits of the lowest series. Finally, there are 

 a few places where fuller sequences with a thick middle series are 

 found. 



