38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 64 



UNCONFORMITY AT BASE OF CAMBRIAN 

 Dr. Bailey Willis has given a very clear and full description of 

 the Sinian system, the lower portion of which is referred to the 

 Cambrian.* He found the unconformity at the base of the Sinian 

 that divided the Paleozoic from the pre-Cambrian to be a break of 

 the first magnitude even where the underlying strata are the Ta- 

 yang (Nan-k'ou) limestone of the late Proterozoic' 



The mechanical sediment of the basal formation has the character of a fine 

 alluvium and is of uniform moderate thickness, 350 to 500 feet, 105 to 150 

 meters. The material is red soil, particles of ferruginous clay being thor- 

 oughly oxidized and grains of sand coated with ferric oxide. The plane 

 of contact at the base is sharply defined, usually very even, not broken by 

 abrupt hollows or decided projections, but swelling gently over rounded bosses 

 of the harder rocks. Pebbles of the subjacent rocks are wanting in the basal 

 deposits, as a rule, and where they occur are limited to very local accumula- 

 tions. Beds of arkose have not been seen, nor even beds of clean sand such as 

 waves usually spread. Thus none of the effects of violent breakers are pres- 

 ent; the evidence is that a gentler agent cleaned the surface of the ancient 

 rocks. The facts support the view that the lowest strata of the Man-t'o for- 

 mation were laid down in the shallows, lagoons, and flood-plains of a very 

 low, flat coast, where weak waves, feeble shore currents, and rivers interacted.' 



In discussing the unconformity at the base of the Sinian, Doctor 

 Willis states that each unconformity is somewhere represented by 

 continuous, conformable deposits, and the area of unconformity is 

 bounded by the areas of conformity : 



When we pass from one to the other there is difficulty in dividing the con- 

 tinuous series of strata at a plane corresponding to that indicated by the 

 discontinuity in the neighboring series. This condition exists at the base of 

 the Cambrian in certain localities in the United States, where the lowest fos- 

 siliferous Cambrian strata are conformably underlain by great thicknesses of 

 sediments, that accumulated in the depressions from which the Cambro- 

 Ordovician epicontinental sea expanded. Such sediments are by some re- 

 garded as pre-Cambrian, by some as the downward extension of the Cam- 

 brian. There is no difference of opinion regarding the base in sections where 

 the unconformity intervenes, as is commonly the case.* 



Since W^illis wrote the above in 1907, I have completed my study 

 of the relations of the Cambrian and pre-Cambrian in North Amer- 

 ica and have concluded that the pre-Cambrian unconformity is uni- 

 versal in all known localities of Cambrian sedimentation and that 

 the depressions in which the pre-Cambrian sediinents were deposited 



'- Willis, Bailey. Research in China, Pub. No. 54 Carnegie Institution of 

 Washington, Vol. 2, 1907, Systematic geology, pp. 35, 49. 

 ^ Idem, p. 31. 

 ' Idem, p. S2. 

 * Idem, p. 35. 



