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GEOLOGY. 

 Silurian in the West. 



The Silurian and Ordovician systems have been usually mapped as 

 one under the former name in the area west of the Great Plains. At 

 various points in this region there is, between the Ordovician below 

 and the Devonian above, a series of sedimentary beds poor in fossils. 

 The character of the fossils being indecisive, the age of the beds has not 

 been certainly determined. Some of them may be Silurian, though 



Fig. 182. — Diagram showing overlaps and succession of formations of the eastern 

 interior sea during the Silurian and Early Devonian periods (Hartnagel.) 



in many regions all appear to be referable to the Ordovician. If they 

 are to be correlated with the Silurian, the Ordovician types of life may 

 have lingered on in the west much longer than in the east. If the Silu- 

 rian is really absent from all the areas where its presence is not known, 

 it would appear that a large part of western North America was 

 land during the Silurian period. The Silurian system has not been 

 identified in the fine sections of the Colorado canyon in Arizona, and 

 probably does not exist there. This may mean that either this part 

 of the west was land during the Silurian period, or that such Silurian 

 formations as were deposited here were subsequently removed by 

 erosion during an interval of emergence. Silurian beds are reported 

 from the Fort Apache region of Arizona, 1 from southern California and 

 Nevada, 2 and from the Seward Peninsula of Alaska. 3 They are doubt- 

 fully reported from the Canadian Rockies, but the beds in question 

 seem to belong to the same system as many other western beds, referred 

 tentatively to the Ordovician on the basis of their fossils. 4 



1 Reagan, Am. Geol., Vol. 32, 1903, p. 278. 



2 Spurr, Bull. 208, U. S. Geol. Surv., pp. 70, 86, and 202, 



3 Collier, Professional Paper No. 2, U. S. Geol. Surv. 



4 Dawson, Bull Geol. Soc. Am., Vol. 12, p. 68. 



