10 p. B. KING 



suspicion that geosynclines were features marginal to the continental 

 platform, that overlapped its edges, it is true, but that, on the farther 

 side, may have been built outward into the ocean basins (Longwell, 

 1950, pp. 420-422). 



The part of the geosyncline toward the continent, termed the 

 miogeosyndine, was a shelf underlain by continental crust that re- 

 ceived various sorts of shallow-water sediments. Mobility of the 

 crust beneath it was greater than that in the continental interior, yet 

 was expressed mainly by subsidence during sedimentation, which 

 permitted gradual accumulation of a thick body of sediments. 



The part of the geosyncline toward the ocean, termed the eugeo- 

 syncline, was a more mobile area, even during early phases of its 

 history, with deeps, shallows, and strips of land that were shaped by 

 crustal forces, and with volcanic eruptions whose products were 

 mostly spread on the sea floor, but which in places were built up 

 into islands. Later, parts of the geosynclinal phase in this area 

 blended w^th the succeeding orogenic phase. The extent to which the 

 eugeosyncline formed over a continental or over an oceanic crust is 

 uncertain, as the basement on which the eugeosynclinal deposits 

 were laid has seldom been raised to the surface; at least the outer 

 edge of the eugeosyncline was probably built over an oceanic area. 



Our study of western North America can best begin at the start of 

 the Mesozoic era, or late in the geosynclinal phase of development 

 of the Cordillera, and immediately before the orogenic phase. 



Eugeosynclinal Area of the Cordillera 



A eugeosynclinal environment persisted for a long period in much 

 of the western part of the Cordilleran region, with an irregular 

 eastern boundary. It extended about to the site of Owens Valley 

 east of the southern Sierra Nevada, east of Winnemucca in north 

 central Nevada, and across Oregon into westernmost Idaho (Fig. 2). 



The environment is expressed by a characteristic suite of deposits 

 — volcanics (lavas, tuffs, and breccias) many times repeated and of 

 great thickness, and associated argillites, graywackes, and cherts, 

 nearly all of which must have been laid down beneath the sea 

 (Eardley, 1947, pp. 316-328). The eugeosyncline originated at an 

 early period, for deposits of eugeosynclinal type contain Silurian 

 and Devonian fossils in the Klamath Mountains and Ordovician 

 fossils in north central Nevada and the eastern Sierra Nevada, but 



