Charles Schuchert — Historical Geology, 1818-1918. 77 



system is older than the New York system, and therefore 

 older than the Hudson River group of Ordovician age, 



In 1888 Walcott presented his conclusions in regard to 

 the sequence of the strata in the typical Taconic area and 

 to the north and south of it. He collected Lower Cam- 

 brian fossils at more than one hundred localities 

 "within the typical Taconic area," and said that the 

 thickness of his "terrane No. 5" or "Cambrian (Geor- 

 gia)," now referable to the Lower Cambrian, is "14,000 

 feet or more." He demonstrated that the Lower Cam- 

 brian is infolded with the Lower and Middle Ordovician, 

 and confirmed Emmons's statement that the former rests 

 upon his Primary or Pre-Cambrian masses. Elsewhere, 

 he writes: "To the west of the Taconic range the sec- 

 tion passes down through the limestone (3) [of Lower 

 and Middle Ordovician age] to the hydromica schists (2) 

 [whose age may also be of early Ordovician] , and thence 

 to the great development of slates and shales with their 

 interbeclded sparry limestones, calciferous and arenaceous 

 strata, all of which contain more or less of the Olenellus 

 . . . fauna. ' ' He then knew thirty-five species in "Wash- 

 ington County, New York (35, 401, 1888). 



Finally in 1915 Walcott said that in the Cordilleran 

 area of America there was a movement that brought 

 about changes "in the sedimentation and succession of 

 the faunas which serve to draw a boundary line between 

 the Lower and Middle Cambrian series. . . . The 

 length of this period of interruption must have been con- 

 siderable . . . and when connection with the Pacific was 

 resumed a new fauna that had been developing in the 

 Pacific was then introduced into the Cordilleran sea and 

 constituted the Middle Cambrian fauna. The change 

 in the species from the Lower to the Middle Cambrian 

 fauna is very great." He then goes on to show that in 

 the Appalachian geosyncline there was another move- 

 ment that shut out the Middle Cambrian Paradoxides 

 fauna of the Atlantic realm from this trough, and all 

 deposition as well. 



Conclusions. — Accordingly it appears that everywhere 

 in America the Lower Cambrian formations are sep- 

 arated by a land interval of long duration from those of 

 Middle Cambrian time. These formations therefore 

 unite into a natural system of rocks or a period of time. 

 Between Middle and Upper Cambrian time, however, 



