200 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



in Idaho, both include thick members which were deposited at no great dis- 

 tance from shore.** 



The land in both cases lay to the westward and in both districts, rocks of 

 Archean habit are well developed to the westward of the areas occupied by the 

 Belt-Cambrian rocks. The western rim of the geosynelinal as representing the 

 zone of old shore-lines may here be tentatively fixed at about 118° west 

 longitude. 



The heavy lava blanket of southern Idaho and southeastern Oregon 

 effectually precludes the discovery of either the rocks or the relations of the 

 early Paleozoic or pre-Paleozoic terranes. There is little doubt, however, that 

 the great geosynelinal once stretched far to the southward and probably with- 

 out serious interruption, into Nevada. The geologists of the Fortieth Parallel 

 survey showed that a great thickness of conformable Cambrian and pre-Cam- 

 brian beds were deposited over the area of what is now the Great Basin of 

 Utah and Nevada. Hague agrees with King that the bulk of the detrital 

 material in these deposits was washed out from a zone of shore-lines, located 

 on the Fortieth Parallel near the meridian of 117° 30', west longitude. For 

 the Belt-Cambrian rocks King places the shore-line zone at 116° 30', west 

 •longitude.f In the Eureka district Hague found 6,250 feet of pre-ITpper Cam- 

 brian strata which represent only the upper part of the geosynelinal, as the 

 base was everywhere concealed.:}: 



In the Wasatch, Walcott describes more than 11,000 feet of beds conform- 

 able to overlying strata bearing the Olenellus zone.* The Uinta quartzite 

 formation, 12,000 + feet thick, underlies the Middle Cambrian Lodore shales 

 quite conformably. Being unfossiliferous, the quartzite is referred to the pre- 

 Cambrian. The lithology is very similar to that of the Purcell series and it is 

 noteworthy that sheets of contemporary lava, analogous to the Purcell Lava, 

 occur in the Uinta quartzite.§ 



In all of these standard sections of the Great Basin geosyncline, the litho- 

 logical character of the sediments corresponds well with that of the many sec- 

 tions now run through the geosyncline near the Forty-ninth Parallel. There 

 is every probability that these northern and southern sections include different 

 parts of the same great unit, the Rocky Mountain Geosynelinal, which thus 

 extended, without sensible interruption, from the Fortieth Parallel to and 

 beyond the Sixtieth Parallel of latitude. 



**U.S. Geol. Surv. Bull. No. 260, 1905, p. 274; Bull. No. 285, 1906, p. 41; and Prof. 

 Paper No. 27, 1904, p. 16. 



fC. King— Geological Exploration of the Fortieth Parallel— Systematic Geology, 

 Vol. 1, 1878, p. 534, and map, p. 127; A. Hague, Geology of the Eureka District, Mono- 

 graph 20, U.S.jGeol. Survey, 1892, p. 175. — The more recent discovery of Lower Cam- 

 brian formations in the White Mountain range of eastern California, shows that, for 

 at least part of Belt-Cambrian time, the shore-line must have been situated west of the 

 limit set by King.— <cf. C. D. Walcott, Amer. J-our. of Science, 3rd ser., Vol. 49, 1895, 

 p. 141. 



I Op. cit. p. 13. 



*C. D. Walcott, Tenth Annual Report, U.S. Geol. Survey, 1890, p. 550. 



§F. B. Weeks, Bull. Geol. Soc. America, Vol. 18, 1907, p. 434. 



