CAMBRIAN AND LOWER ORDOVICIAN. 81 



112. ARIZONA, a 



The Tonto group is recognized by Reagan/"^ who states: 



The Tonto formation is composed of coarse to fine grained, often cross-bedded vitreous 

 sandstones and sandy shales, varying in color from brown to red, purple, and white. The forma- 

 tion is the surface rock of a large area. The Sierra Ancha Mountains are capped with it. It is 

 exposed aU around the rim of the Ton to-Cherry Creek basin and continues on the east from that 

 basin beyond the headwaters of Oak Creek in the latitude of Chiddessky on Canyon Creek. 

 From this point it extends in an ever-widening strip along the plateau south across Salt River 

 to the Apache and Nantan mountains. Patches of it also occur in the Apache and Pinal 

 mountains. In short, the Tonto exposures completely encircle the Archean and Algonkian 

 areas of the Salt River and Canyon Creek region. That it once covered the entire Archean and 

 Algonkian area is attested by the vitreous sandstone points and buttes scattered over the 

 country, among which are points Chiddesche and the Twin and Sombrero buttes. 



I 13. NEW MEXICO. 



Lindgren'*^* gives the following account of the Cambrian and Ordovician of 

 New Mexico: 



The pre-Cambrian history of the Territory, though imperfectly known, indicates a period 

 of sedimentation followed by mountain building and igneous intrusion, which was in turn 

 succeeded by long-continued erosion that exposed the intrusive cores. At the base of the 

 Paleozoic section is a strongly pronounced unconformity. In general it has a fundamental 

 appearance, as, for instance, where sandstones and limestones rest horizontally on vertical 

 gneisses. 



In the whole of northern New Mexico the Pennsylvanian or upper Carboniferous rests 

 directly on the pre-Cambrian, and these conditions appear to continue as far south as Socorro. 

 Cambrian rocks were first definitely recognized in New Mexico in 1905 by C. H. Gordon,'' who 

 found fossils of this age in the Caballos Range, along the thirty-third parallel. The thickness 

 of the Cambrian quartzite and shales, which here underlie the Ordovician limestone, is only 

 65 feet. In the Mimbres Mountains, near Kingston, the thickness is 75 feet, and in the Florida 

 Range, southeast of Deming, 135 feet of quartzites were measured in the same stratigraphic 

 position. Toward the west, near Silver City, L. C. Graton measured a thickness of nearly 

 1,100 feet of quartzitic sandstones, including some strata of limestone. Across the Arizona 

 line in the same latitude Waldemar Lindgren " found 280 feet of quartzites, with some shales, 

 resting on granite underneath the Ordovician and obtained near the top of these rocks some 

 fossils suggestive of the Cambrian. In the Franklin Mountains G. B. Richardson <* found 150 

 feet of sandstone containing Upper Cambrian fossils. At Bisbee, in southern Arizona, F. L. 

 Ransome measured 400 feet of nonfossiliferous, probably Cambrian sandstone, covered by 

 750 feet of fossihferous Middle Cambrian limestone. This is the only known occurrence of 

 heavy Cambrian Hmestone in this region, although it is possible that some of the Umestone 

 which in southwestern New Mexico covers the quartzite may also be of Cambrian age. 



All this indicates beyond doubt that during the period preceding the Cambrian the whole 

 of western New Mexico, so faj- as available exposures permit a judgment, was a land area which 

 gradually became planed down from a very high to a more moderate rehef. During the Cam- 

 brian period the sea appears to have advanced northward, but only to about latitude 33° 30', 

 where the Cambrian deposits seem to thin out. The greatest thickness of sediments is found 



a See also I-J 12, Grand Canyon, Arizona (pp. 88-89). 



6 Gordon, C. H., and Graton, L. C, Lower Paleozoic formations in New Jlexico: Am. Jour. Sci., 4th ser., vol. 21, 

 1906, pp. 390-395. 



c Clifton folio (No. 129), Geol. Atlas U. S., U. S. Geol. Survey, 1905. 



d Tin in the Franklin Mountains, Texas: Bull. U. S. Geol. Survey No. 285, 1906, pp. 146-149. 



48011°— 12 6 



