Page:Walcott Cambrian Geology and Paleontology II.djvu/31

 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. Subsequent 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 Olenellus, 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 Carolina, 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 Linnville 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.

In my paper on "The North American Continent during Cambrian Time" attention is called to a series of conformable pre-Cambrian rocks found in the Appalachian and Rocky Mountain troughs [Walcott, 1892, p. 544] that were thought to be conformably beneath the Lower Cambrian sandstone. Mr. Keith's detailed work, as cited above, has proven the presence of a marked unconformity in the southern Appalachian area. During the past ten years, as incidental to my Cambrian work, I have been studying the contact between the Cambrian and pre-Cambrian in the Cordilleran area. From