82 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



at Silver City. In the Caballos Range there appears to exist a gradual transition between 

 quartzitic sandstone and underlying granite, suggestive of secular decay of the granite, 

 preceding deposition. 



It is interesting to note that the Ordovician, Silurian, Devonian, and lower Carboniferous 

 (Mississippi an) are likewise lacking in northern New Mexico, and that they begin to appear 

 prominently at about latitude 33° 30'; some exposures of the Mississippian have been recog- 

 nized at about the latitude of Socorro, or 34°. * * * 



The Ordovician rests conformably on the Cambrian and consists of 450 to 1,200 feet of 

 usually cherty limestone, very poor in fossils. It has been recognized in the Caballos Range, 

 in the Mimbres Range, and at Silver City and other places. 



I 14-15. OKLAHOMA. 



In the Arbuckle Mountains of Oklahoma the Upper Canlbrian and Lower 

 Ordovician are represented by the Reagan sandstone and Arbuckle limestone. 

 Taff ^"^ gives the following descriptions: 



In pre-Cambrian [late Cambrian] time the region of the Arbuckle uplift was submerged, 

 and the Reagan sandstone, the lowest Cambrian sediment, was laid down upon the eroded 

 surface of the granite and porphyry as a beach and off-shore deposit. The pre-Cambrian 

 land, composed of igneous rocks, possessed some rehef at the time of submergence, as is shown 

 by the uneven contact of the igneous rocks -vyith the Reagan sandstone and also by the variable 

 aature of the sandy deposits. The land in the immediate vicinity of the uplift was soon sub- 

 merged, however, as is shown by the presence of fossiliferous horizons representing the same 

 mid-Cambrian time in the upper member of the Reagan sandstone wherever it is exposed. 



CAMBRIAN EOCKS. 



Reagan sandstone. — This sandstone is a variable formation, as regards both its thickness 

 and the nature of its materials. In the eastern part of the uplift, in contact with the granite, 

 it varies in thickness from a few thin beds to approximately 500 feet, with an average thickness 

 of about 300 feet. In its thickest portion, at the western and of the granite, the lowest beds 

 are composed for the most part of coarse arkose grit. The following is a record of the thickest 

 section of the Reagan sandstone as it occurs at the western end of the granite area, beginning 

 at the base: 



Section of Reagan sandstone. 



Feet. 



Quartzite and arkoee conglomerate, composed of poorly sorted granitic materials.- 30 



Coarse grit and sand, with some clay and green sand in upper part, generally well stratified 370 



Thia-bedded and laminated sandstone, becoming calcareous ia the upper part 60 



This section is followed by generally thin-bedded siliceous Umestone and shaly strata 

 through several hundred feet to the top of the middle Cambrian. The calcareous beds at the 

 top and above the Reagan contain numerous fossils of weU-defined middle Cambrian forms. 



The Reagan sandstone overlying the porphyry in the southern sides of the East and West 

 Wooded hills has a section similar to that in contact with the granite. The coarse materials 

 which make lentils of conglomerate near the base, however, are here composed of porphyry 

 and associated dike rock. In the upper part of the formation greenish clays are probably 

 more abundant, and the green sand continues higher up into the more calcareous beds. As 

 in the occurrence near the granite, here the calcareous rocks contain numerous middle Cambrian 

 fossils. 



CAMBEO-OEDOVICIAN KOCKS. 



Arbuckle limestone. — The Arbuckle limestone, with the exception of thin, shaly strata 

 and occasional siliceous and cherty beds, is composed entirely of light-blue and white limestone 

 and cream-colored to white crystaUine dolomite. The section ranges in thickness from 4,000 



