282 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 64 



In view of the data and discussion already presented I do not think 

 it necessary to comment further upon the two sections. 



UNCONFORMITY BETWEEN THE CAMBRIAN AND PRE- 

 CAMBRIAN 



Having- conchided from stratigraphic and paleontologic evidence 

 that there was no unconformity or discordance between the Cambrian 

 and pre-Cambrian at Helena, Rothpletz next proceeds to argue that 

 because the evidence of non-conformity is not readily seen at Helena, 

 Montana, it is quite probable that it does not exist. It was quite 

 natural that he did not discover it about Helena for two reasons. 

 First, because it is not very clearly shown. Second, he was looking 

 for it within the Cambrian section and not below the Cambrian and 

 above the Helena limestone of the pre-Cambrian (see pp. 279-281). 



Grand Canyon section. — If Rothpletz had studied the section at the 

 Grand Canyon of the Colorado River when there in 1913,^ he might 

 have obtained a very clear conception of the unconformity between 

 the Cambrian and pre-Cambrian. In that section ^ the entire pre- 

 Cambrian sedimentary series of nearly 12,000 feet of sandstones, 

 shales, and limestones were tilted, faulted, more or less broken, and 

 eroded nearly to base level before the transgressing Cambrian sea 

 covered it with beach gravel and sands, arenaceous and calcareous 

 muds in which fragments of an abundant Middle Cambrian fauna 

 were buried. This great unconformity has been illustrated and 

 described several times,' but Rothpletz does not refer to it in his 

 memoir, although it is the one great locality where Nature has laid 

 open the history of a great, unmetamorphosed, sedimentary series of 

 pre-Cambrian formations corresponding in stratigraphic position and 

 character with the Belt series of Montana. As giving some concep- 

 tion of the unconformity, a section is inserted here (fig. 11) showing 

 the unconformity between the Cambrian Tapeats sandstone and the 

 pre-Cambrian Grand Canyon series of formations. Four photo- 

 graphs taken in the Grand Canyon are also reproduced in which the 

 unconformity is finely shown (pis. 40-43). 



Bow River Valley section. — Rothpletz refers to my description * 

 of the Bow River section of Alberta where the Lower Cambrian is 



^ Rothpletz, p. 4. 



-Walcott, Fourteenth Anti. Rept., U. S. Geol. Surv., 1895, pp. 507-519. Idem, 

 Bull. Geol. Soc. America, Vol. i, 1889, pp. 51-56, figs. 1-9. 

 ^ Powell, Exploration Colorado River of the West, 1875, p. 212, fig. 79. 

 Walcott, 1895, Fourteenth Ann. Rept, U. S. Geol. Surv., pp. 507-5^7- 

 Noble, Bull. U. S. Geol. Surv., No. 549, 1914, p. 65, plates 9 and 18. 

 ^ Smithsonian Misc. Coll., Vol. 53, 1910, p. 426. 



