30 UNCONFORMITY AT BASE OF CAMBRIAN. 



he says that the presence of a rich fauna in the Sinian seas distinguishes 

 that period from the preceding (Hu-t'o) time. It was the absence of marine 

 life and the character of the sediments that led me to conclude that there were 

 no marine deposits on the North American continent (nor probably on any 

 of the continents) representing the Lipalian interval or the interval between 

 the fossiliferous Cambrian formations and the period of the development of 

 the early pre-Cambrian marine life along the shores of the continents. [Wal- 

 cott, 1910/1, pp. 14-15 1 



I now anticipate that if the rocks of the Wu-t'ai and Hu-t'o systems are 

 studied with the view that they may not be of marine origin, they will be 

 found to have been deposited as epicontinental sediments accumulated on 

 flood plains or in bodies of fresh water. In part they are more altered and 

 metamorphosed than the pre-Cambrian sedimentary rocks of North America, 

 and hence it may be more difficult to determine their origin. 



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 [Willis, 1907, 

 pp. 35-49]. 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 magni- 

 tude even where the underlying strata are the Ta-yang (Nan-k'ou) limestone 

 of the late Proterozoic [Willis, 1907, p. 31]: 



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 thoroughly 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 accumulations. 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 present ; 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 formation 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. [Willis, 1907, p. 32.] 



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

 states that each unconformity is somewhere represented by continuous, con- 

 formable deposits, and the area of unconformity is bounded by areas of 

 conformity : 



When we pass from one to the other there is difficulty in dividing the continuous 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 fossiliferous 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 regarded as pre- 

 Cambrian, by some as the downward extension of the Cambrian. There is no difference 

 of opinion regarding the base in sections where the unconformity intervenes, as is commonly 

 the case. [Willis, 1907, p. 35.] 



