94 A CENTURY OF SCIENCE 



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 

 interbedded 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, 

 there appears to be a complete transition in the Cordil- 

 leran trough, binding these two series of deposits into 

 one natural or diastrophic system. Hence the writer 

 proposes that the Lower Cambrian of America be known 

 as the Taconic system. The Middle and Upper Cam- 

 brian series can be continued for the present under the 

 term Cambrian system, a term, however, that is by no 

 means in good standing for these formations, as will be 

 demonstrated under the discussion of the Silurian con- 

 troversy. 



