NO. I ABRUrX APPEARANCE OF THE CAMBRIAN FAUNA II 



of the Cambrian and pre-Cambrian contact in eastern Tennessee, 

 who says ' : 



With the deposition of the Cambrian rocks there came a great change in 

 the physical aspect of this region. The sea encroached on areas which for a 

 long time had been dry land. Erosion of the surface and eruptions of lava 

 were replaced by deposition of sediments beneath a sea. Extensive beds of 

 these rocks were laid down in some areas before other areas were submerged, 

 and the sediments lapped over lavas and plutonic granites alike. The waste 

 from them all was combined in one sheet of gravel and coarse sand, which 

 now appears as shale, sandstone, conglomerate, and rocks derived from them. 

 The thickness of this first formation varies greatly and abruptly in this region, 

 showing that the surface upon which it was laid down was irregular. Subse- 

 quent formations of Cambrian age came in a great group of alternating shale 

 and sandstone followed by an immense thickness of limestone and shale. 

 Fossils of Cambrian age, mainly Olenclliis, are found as far down as the 

 middle of the sandstone group. The strata lying beneath the fossiliferous 

 beds differ in no material respect from those overlying. All are plainly due 

 to the same causes and form part of one and the same group, and all are 

 closely associated in area and structure. 



When speaking of the similar contacts in northwestern North 

 CaroHna, he says ^: 



Here the sediments lapped over lavas and plutonic granites alike, and the 

 waste from them all was combined in one sheet of gravel and coarse sand 

 which now appears as sandstone, conglomerate, and quartzite. Some of this 

 waste consists of epidote and jasper, the products of alteration in the Linn- 

 ville metadiabase. It is thus seen that the interval between the Algonkian 

 and Cambrian was at least long enough to permit dynamic movements and 

 chemical changes to effect considerable results, even before the period of 

 erosion and reduction began. 



UNCONFORMITY BETWEEN THE CAMBmAN AND 

 PRE-CAMBRIAN 



In my paper on " The North American Continent during Cam- 

 brian Time " attention is called to a series of conformable pre-Cam- 

 brian rocks found in the Appalachian and Rocky Mountain troughs 

 [Walcott, 1892, p. 544] that were thought to be conformably be- 

 neath the Lower Cambrian sandstone. Mr. Keith's detailed work, 

 as cited above, has proven the presence of a marked unconformitv in 

 the southern Appalachian area. During the past ten years, as inci- 

 dental to my Cambrian work, I have been studying the contact be- 

 tween the Cambrian and pre-Cambrian in the Cordilleran area. From 



' U. S. Geol. Survey, Geological Atlas, Roan Mountain Folio, No. 151, 1907, 

 p. 4. 

 ■ U. S. Geol. Survey. Geological Atlas, Cranberry Folio. No. 90, 1903. p. 4. 



