322 GEOLOGY. 



carry a larger body of sediment to the sea, or that conditions favored 

 the transportation of the mud farther from the shores than formerly. 

 The fineness of the sediment would seem to indicate that the streams 

 were rather sluggish, though locally the sediments were coarse. Asso- 

 ciated with the Upper Ordovician shales, there are considerable bodies 

 of limestone in some places, and of sandstone in others. 



It is perhaps worthy of note that all the Ordovician formations 

 of the interior and the east, unless it be the oldest, bear within them- 

 selves distinct evidence of shallow water origin, and even the excepted 

 formation contains no evidence that it was laid down in deep water. 

 Its paucity of fossils has led to the conjecture that its material may 

 have been largely precipitated from solution. 



Western sections. — In the Great Plains, the Ordovician system 

 appears at the surface but rarely, though it probably underlies the 

 younger formations. West of the Great Plains, the Ordovician sec- 

 tions, so far as now known, are simpler than in the interior or the east. 

 In some places the whole system seems to be represented by a part 

 of a single formation of limestone. Thus in Nevada 1 a part of the 

 Pogonip formation seems to be the only representative of the period. 

 Limestone was probably in process of formation in other parts of the 

 Great Basin and in western Colorado 2 at the same time. In the 

 Rockies sand was the principal deposit in some places 3 and limestone 

 in others. 4 The sections of the Ordovician in the Appendix, vol. Ill, 

 give some conception of its range in the west. 5 



Igneous rocks. — Igneous rocks of Ordovician age attain little impor- 

 tance in North America. Their general absence is in harmony with the 

 general quiet which characterized the period. Igneous rocks of limited 

 extent, which may date from the close of the Ordovician, are found 

 at a few places in New York (notably Cortland County). 



General Conditions and Relations of the Ordovician System. 



Position of beds. — It has already been indicated that the Ordo- 

 vician rocks are in general conformable on the Cambrian, and from 



1 King, Geology of the 40th Parallel, Vol. I. For notes on the formations of Nevada 

 south of the 40th Parallel, see Spurr, Bull. 208, U. S. Geol. Surv. 



2 Anthracite-Crested Butte and Ten Mile (Colo.) folios, U. S. Geol. Surv. 



3 Pike's Peak (Colo.) folio, U. S. Geol. Surv. 



4 Absaroka and Yellowstone National Park (Wyo.) folios, U. S. Geol. Surv. 

 5 In addition 1 to reports and folios already mentioned, the Ordovician (Silurian) 

 system is shown on the Lassens Peak (Cal.) folio, U. S. Geol. Surv. 



