Genem- Paleozoic and Mesozoic-Cenozoie, 473 



practically begins, and the authors deal with the records especially 

 as they are exemplified on the North American continent. The 

 principles which they seek to teach are those applicable to all 

 continents, one object being " to combine dynamical discussion with 

 the phenomena which it is to explain, as these phenomena unfold 

 themselves stage by stage." Thus they deal not only with the 

 local characters of the strata, their subdivisions and their distribution 

 in other areas, but with their physiographic relationships, the 

 features of ancient lands, with sources of sediments, and their mode 

 of derivation. They treat also of the permanence of oceans and 

 continents, and of periodicity in great deformative movements. 

 Evolution and the distribution of faunas and floras are discussed 

 apart from the stratigraphy. 



Many interesting comparisons are made and suggested in the 

 course of the work. No distinct physical or organic break is 

 recognized between Cambrian and Ordovician. The Old Red 

 Sandstone is regarded as a phase of the Devonian, some portions 

 of it not improbably having been of suhaerial rather than subaqueous 

 origin. Both phases are represented in America. 



it is noteworthy, with regard to the basal Devonian, that there is 

 a difference of opinion as to whether the Helderberg formation 

 should be classed as Silurian or Devonian. It has a strong Silurian 

 facies, but the species, as now known, are all new, except Leptcena 

 rJiomhoidalis and Atrypa reticularis — the " Methuselahs of Paleozoic 

 times." The authors remark that " the Helderberg formation is 

 largely limestone, which suggests that subaerial erosion was not 

 taking place at a rapid rate, and that the principal formation of the 

 epoch, as far as our continental area is concerned, was extracted from 

 the sea-water." The question of the mechanical derivation of 

 calcareous mud is not apparently taken into consideration. 



It is interesting to compare the latest stages of the Carboniferous, 

 locally "Upper Barren Coal Measures," with approximately 

 equivalent strata in England. Great movements were likewise 

 inaugurated at about the close of the Carboniferous period in 

 America. 



The Permian, " often regarded as a part of the Carboniferous," 

 was formed in waters either in the form of lakes and inland seas or 

 in connection with the open ocean. The Perino-Carboniferous 

 glaciation in Australia, India, and Africa is spoken of as " the first 

 well deciphered glaciation," though mention is made of that in the 

 ■Cambrian or pre-Cambrian era. The prevalent red sandstones in 

 the Newark Series show points in common between the Trias ia 

 America and England. The Eh^etic beds (grouped with the Trias) 

 have not been recog-nized in America. Jurassic formations have 

 jiowhere been determined in the eastern half of the continent, but m 

 western America they occur in nearly the same areas as the Trias. 

 'The earliest Liassic fauna is not derived from the locally preceding 

 forms of life, but has the aspect of European Lias, and of the Liassic 

 fauna in Timor Island and in Argentina. In the higher stages 

 Aucella and other forms connect with the Upper Jurassic of Russia. 



