NO. 4 CAMBRIAN AND PRE-CAMBRIAN AT HELENA 289 



Cambrian and pre-Cambrian. This marked unconformity is the record of the 

 advancing-, overlapping Lower Cambrian sea of southwestern Nevada, the 

 Middle Cambrian sea of Utah and Idaho, and finally the Upper Cambrian sea 

 of Colorado and the interior continental area. 



The Cambrian rocks may be abruptly conformable upon the xA.lgonkian or 

 Archaean,^ or apparently conformable, as in areas where there has been very 

 little disturbance of the subjacent Algonkian beds.^ Over the interior of the 

 continent the Upper Cambrian strata unconformably overlap the Algonkian 

 and Archaean,^ and there is here .no record of any part of the Lower Cambrian 

 period. I do not know of a case of proven conformity with transition deposi- 

 tion between Cambrian and pre-Cambrian Algonkian rocks on the North 

 American continent. In all localities where the contact is sufficiently exten- 

 sive, or where fossils have been found in the basal Cambrian beds or above 

 the basal conglomerate and coarser sandstone, an unmistakable hiatus has 

 been found to exist. Stated in another way, the pre-Cambrian land surface 

 was formed of sedimentary, eruptive, and crystalline rocks, the deposition of 

 which did not in any known instance immediately precede the Cambrian sedi- 

 ments. Everywhere there is a marked stratigraphic and time break between 

 the known pre-Cambrian rocks and the Cambrian strata of the North Ameri- 

 can continent.* 



The Lower Cambrian is characterized in southwestern Nevada by the pres- 

 ence of the Lower Cambrian" (Waucobian)" fauna, which there ranges 

 through some 4,000 feet (1,220 m.) of strata that have no known line of demar- 

 cation at the base to separate the Cambrian from some pre-Cambrian Paleo- 

 zoic formation. This leads to the hope that still older beds and faunas will be 

 discovered in this region which will establish a base to the Cambrian not 

 entirely founded on unconformable superposition of the Cambrian on pre- 

 Cambrian formations. 



In mentioning the pre-Cambrian surface I said : ' 



From the evidence afforded by the stratified rocks and their contained fossils, 

 the first known sediments were deposited in a shallow marine basin that 

 occupied an area now included in southwestern California and adjacent por- 

 tions of Nevada. The incoming Cambrian sea encountered a land surface 

 deeply disintegrated and more or less eroded nearly to base-level. Compared 

 with the earlier epochs of Algonkian time it was a featureless surface, the 

 elevations caused by folding and uplift in the geosynclines and the adjoining 

 geanticlines of the Cordilleran, Lake Superior, and Appalachian areas of 

 Algonkian time having been largely degraded. The rising waters met with 

 only slight elevations in the Cordilleran trough, as evidenced by the almost 

 entire absence of coarse conglomerates and the presence, above the coarse 



^ Tenth Ann. Rept., U. S. Geol. Surv., 1891, p. 551, fig. 48. 

 '^ Bull. Geol. Soc. America, Vol. 10, 1899, pp. 210-213. 

 ^ Tenth Am|. Rept., U. S. Geol. Surv., 1891, pi. 44, pp. 561-562. 

 * Twelfth^^n. Rept., U. S. Geol. Surv., 1892, pp. 546-557. 

 ° Tenth Ann. Rept., U. S. Geol. Surv., 1891, pp. 515, 549. 

 '* Smithsonian Misc. Coll., Vol. 57, 1912, p. 305. 



' Problems of Geology, Yale LTniv. Press, 1914, Chap. 4 : The Cambrian and 

 its Problems in the Cordilleran Region, p. 167. 

 3 



