3 2 GEOLOGICAL BIOLOGY. 



system was too low, including rocks later recognized to be 

 older than the Cambrian system (the " Lower Cambrian 

 group " of 1835), and too high, in that the " Upper Cambrian 

 group " of this first paper was claimed also by Murchison in 

 his original Silurian system ; and in fact the Upper Cam- 

 brian of Sedgwick is the same stratigraphically with the 

 Lower Silurian of Murchison, as at present used. The Cam- 

 brian system includes the " Middle Cambrian" of the 1835 

 paper, which is composed of the following formation, viz. : 

 Longmynd, Harlech, Menevian, Lingula flags, and Tremadoc, 

 The rocks at first were believed to contain no fossils ; later, 

 fossils were found, and were more fully elaborated in Bohemia 

 by Barrande, and defined by him as the " first fauna." In 

 later correlations, and in other countries this first or primor- 

 dial fauna of Barrande has been the distinguishing evidence of 

 the Cambrian period of time. 



The Cambrian system in North America includes three 

 divisions: the earliest, or lowest, the (i) Georgian group, typi- 

 cally represented in the shales and limestones of that name in 

 Western Vermont, and containing a fauna characterized by 

 the presence of the Olenellus, a genus of Trilobites. The 

 second or middle division is the Acadian group, typically 

 seen in the form of shales and slates in Eastern Massachusetts, 

 in New Brunswick, and in Newfoundland, and containing the 

 Paradoxides fauna, or the fauna with the genus Paradoxides. 

 The third division is the Potsdam group, and is typically 

 represented in sandstones about the base of the Adirondack 

 mountains, and contains the genus Dicellocephalus. 



Ordovician System. The ORDOVICIAN SYSTEM is a name 

 proposed by Lapworth, in 1879, as a substitute and compro- 

 mise for the Upper Cambrian of Sedgwick and the Lower 

 Silurian of Murchison, both of which covered the same inter- 

 val, and the original usage of which in current geological 

 literature the geologists of the two schools have, since the 

 death of the authors, strenuously maintained. The standard 

 series of rocks are in Wales and Western England, and are the 

 Arenig, Llandeilo flags, and Bala or Caradoc. The fauna is 

 the " second fauna " of Barrande, and the standard system in 

 North America includes the Calciferous group, typically 



