CAMBRIAN AND LOWER ORDOVICIAN. 89" 



The base rests unconformably upon the Chuar shales, etc. Nos. 4 and 5 are referred to 

 the Middle Cambrian, and the strata above to the Upper Cambrian. 



In 1883 Walcott^^ wrote: 



The geologic age of the Tonto group is determined by the presence of numerous specimens 

 representing the genera Cruziana, Lingulepis, Iphidea, ConocephaJites, Crepicephalus and 

 Dicellocephalus that occur at various horizons in the upper 700 feet of sandstone, shales aiid 

 limestone. No fossils were obtained from the coarse sandstone forming the lower 300 feet. 

 The fauna above, however, shows close relaitions to that of the Potsdam sandstone horizon of 

 central Nevada, the Mississippi Valley, and Saratoga County, N. Y. 



The base line of the Tonto is quite uniform and rests unconformably on the varied strata 

 beneath; here and there a knoll, point or ridge is seen on the pre-Tonto surface that rises nearly 

 through the massive Tonto sandstones that were deposited against and over them, the sea 

 breaking off, and burying with the drifting sand, fragments of the rocky islands. 



The great unconformity beneath the Tonto has been described by Prof. J. W. Powell, who 

 examined it in his boat trips down through the Grand Canyon, and by Capt. C. E. Dutton, who 

 viewed it from the summit of the Kaibab Plateau, 5 miles away. Prof. Powell estimated the 

 strata beneath the Tonto and above the Archean at 10,000 feet, and as this is all cut across the 

 unconformity is very great. A detailed study adds to the thickness of the strata; it shows that 

 the original summit of the pre-Tonto group had undoubtedly been cut away more or less before 

 the deposition of the Tonto group ; that the plane of erosion cut deeply into the Archean, and ' 

 that besides the 13,000 feet of strata, that have been planed off, of which the record is found in 

 the section preserved, there was also a problematical amount of considerable thickness. 



I-J '16-17. TENNESSEE AND NORTH CAROLINA. 



In eastern Tennessee and North Carolina two distinct sequences of strata are 

 assigned to the Cambrian. One consists of the sparsely fossiliferous siliceous group 

 to which Safford applied the name ChiUiowee sandstone but which has been sub- 

 divided by Keith into several formations, and the overlying richly fossiliferous, gen- 

 erally calcareous deposits which constitute the Apison, Beaver, Rome, Conasauga, 

 and Knox formations and some minor divisions of local extent; these are found in 

 the valley region of east Tennessee. The other sequence is the more or less meta- 

 morphosed unfossiliferous Ocoee group of Safford, composed in part of coarse clastic 

 deposits with slate and subordinate limestone, which form large masses of the 

 Great Smoky Mountains. Keith ^^ has demonstrated the conformity of Safford's 

 Ocoee to the fossiliferous Cambrian and classes it as Cambrian on grounds of 

 physical relationship. He says: 



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

 aspect of this region. The sea encroached upon areas which were until then dry land. Erup- 

 tions of lava and erosion of the surface were replaced by deposition of sediments beneath a sea. 

 Extensive beds of these were laid down in some areas before others were submerged. Here the 

 sediments lapped o^^er lavas and plutonic granites ahke, and the waste from them all was com- 

 bined 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 LinvOle meta-diabase. It is thus seen that the interval between the Algonkian and Cam- 

 brian was at least long enough to permit dynamic movements and chemical changes to effect 

 considerable results, everi. before the period of erosion and reduction began. 



The two sequences generally do not occur in their original relations, which are 

 therefore indeterminate over wide areas. The Ocoee of Safford is usually over- 

 thrust from the east upon younger strata to an extent which obliterates the normal 



