244 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1915. 



the Colorado Eiver, Arizona.^ In this section the Cambrian strata 

 extend down to the horizon of the central portion of the Middle 

 Cambrian (Acadian) where the Cambrian rests unconformably on 

 the pre-Cambrian formations.- 



The object of my preliminary correlations of the several sections 

 studied, was to show in a broad way the interrelations of the strata 

 and faunas in the North American Cordilleran area w^est of the great 

 continental land area of Lower and much of Middle Cambrian time. 



In the course of my studies, particularly in recent years, data haA^e 

 also come to light which help us more definitely to outline the boun- 

 daries of the three great marine incursions of Cambrian time. There 

 are also presented to us new conceptions of geological conditions in 

 that period and more accurate information indicating the probable 

 sources of the Cambrian fauna of the Cordilleran area.^ 



The change in the species from the Lower to the Middle Cambrian 

 fauna is very great.* Of 77 species of brachiopods in the Lower 

 Cambrian, six are found in the Middle Cambrian. Among the trilo- 

 bites the disappearance of the Mesonacida;^ is the most marked 

 change. Some of the species of the Conocephalida3 may have con- 

 tinued on into the Middle Cambrian, but the study of this and other 

 crustaceans of the Cambrian time has not yet advanced so that any 

 reliable data are available. 



Most of the genera of the Lower Cambrian pass up into the Middle 

 Cambrian, and this leads to the tliought that the interruption, though 

 important and of considerable duration, was not of a degree com- 

 parable with the unconformity immediately preceding the pre-Cam- 

 brian revolution, nor like the great faunal change that came at the 

 close of Cambrian time, although the later diastrophic movement 

 appears to have been relatively insignificant on the western side of 

 the continent. 



After the close of Middle Cambrian time the waters of the Pacific, 

 the Gulf of Mexico, and the Atlantic began to rise and to flood lands 

 that had not known the presence of marine Avaters since far back in 

 the Proterozoic and may be since Archeozoic time. The margin of 

 this area was as far westward as the present position of the main 

 range of the Wasatch Mountains in the vicinity of Salt Lake, Utah ; 

 from this point the shore-line trended gradually south-southwest to 

 southwestern Utah. 



1 Tenth Ann. Rep. U. S. Geol. Survey, 1891, pp. 500-774 : The fauna of the Lower 

 Cambrian or Olenellus zone. 



Smithsonian Misc. Coll., vol. 53, no. 5, 1908, p. 1G7. Cambrian sections of the Cordil- 

 leran area. 



2 See American Jour. Scl., 3(1 ser., vol. 26, 1883, pp. 437-442. 



2 The Cambrian and its problems, Yale Univ. Press, in Problems of American Geology, 

 1915, p. 162. 



*Idem, pp. 189-190. 



^ See plate 14, Lower Cambrian trilobites, facing p. 252, this paper. 



