PHYSICAL AND FAUNAL EVOLUTION 47 



rock which elsewhere carries them, indicates some pecuharity of the 

 sea-shore at that point, capable of barring the life of the sea, is a 

 laborious explanation to fit a preconceived notion of the origin of the 

 formation in question. Nor must we forget that the North American 

 continent was above the sea during long periods of pre-Cambric and 

 Cambric time, and that on those vast land areas subaerial deposition 

 as well as erosion must have been in progress. It is therefore to be 

 expected that in many, if not in most, regions the Paleozoic series 

 begins with a formation of continental origin, the upper portion of 

 which was reworked by the transgressing sea, and became incor- 

 porated as a basal member of the marine series succeeding. In 

 this manner the contact between the continental and marine series 

 often became an apparently conformable and perfectly gradational 

 one, the hiatus between them being masked. It will of course be 

 impossible in such a case to determine whether a basal bed of con- 

 tinental origin is of pre-Cambric, of Cambric, or of post-Cambric 

 age; all that can be determined is the period at which its upper 

 portion was reworked by the transgressing sea. If the basal bed is 

 of slight thickness it is in such a case best referred to the age of the 

 immediately succeeding marine formation. 



The question naturally arises, should the lower portion of the 

 Beekmantown be referred to the Cambric with which it forms a 

 continuous transgressive series, or should it be retained in the Ordo- 

 vicic with the remainder of the Beekmantown ? While in New York 

 the fauna is, so far as known, an Ordovicic one, in other localities 

 beds considered of the same age carry a mixed Cambric and Ordo- 

 vicic fauna. In this respect these beds and the typical Saratogan, 

 as well as the St. Croix series of Minnesota and Wisconsin, probably 

 correspond to the Tremadocien of Europe, which is classed as Upper 

 Cambric by British geologists, but by German and other continental 

 geologists as basal Ordovicic (Unter Silur) . Matthew correlates these 

 beds with the Asaphellus homjrayi beds of the St. John section, 

 and so places them above the Dictyonema flahellijorme beds, which 

 at present are also included in the base of the Ordovicic by some 

 continental geologists. That such transitional formations are to be 

 expected in any complete depositional series is, of course, obvious, 

 and their precise reference is a matter of secondary importance. 



