74 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



The Bolsa quartzite is constant in its lithological character and is readily recognized wherever 

 it occurs. The base of the formation is exposed at many places along Escabrosa Ridge. It is inva- 

 riably marked by a bed of conglomerate from 6 inches to a foot in thickness, resting upon the 

 eroded edges of the pre-Cambrian schists. Most of the pebbles of this basal conglomerate are 

 composed of white vein quartz and are rarely over 3 inches in diameter. This conglomerate 

 is overlain by hard pebbly grits in beds from 10 to 20 feet in thickness, the change from con- 

 glomerate to grit being, as a rule, not very definitely marked. The scattered pebbles in these 

 thick-bedded grits are usually white quartz, but their matrix frequently contains abundant 

 small fragments of pink feldspar mingled with the predominant quartz grains. Cross-bedding 

 is often a conspicuous feature in the lower part of the formation. The pebbly grits in turn 

 pass upward into thinner bedded, more vitreous, fine-grained quartzites showing no feldspathic 

 material, which are conformably overlain by the Abrigo limestone. 



■ The Bolsa quartzite was deposited during an advance of the sea over a subsiding base- 

 ment of pre-Cambrian crystalline rocks which had been worn down by long-continued erosion 

 to a nearly level plain. The arkose character of some of the lower beds of the quartzite shows 

 that this ancient crystalline basement is composed in part of granitic rocks. None of the 

 latter, however, are exposed in the Bisbee quadrangle, where fine-grained sericitic schists are 

 the only pre-Cambrian rocks known. The basal conglomerate of the Bolsa formation is a lit- 

 toral deposit made up of the most durable materials of this schist terrane, chiefly fragments 

 of quartz derived from the numerous irregular veins that occur within the schists, while the 

 finer particles of quartz and flakes of hnca were probably transported by currents into deeper 

 water and contributed to the formation of the overlying quartzite beds. The general nature 

 of the Bolsa formation, particularly the pebbly and cross-bedded character of its lower portion, 

 indicate that deposition took place in comparatively shallow water, with probably a gradual 

 increase in depth as the upper beds were laid down. The conditions were apparently not 

 favorable to marine life, as no organic remains have yet been found in the Bolsa quartzite. 



As no fossils have been found in the Bolsa quartzite its exact geological age is not directly 

 determinable. It will presently be shown, however, that the Bolsa formation is, conformably 

 overlain by the Abrigo hmestone which contains a middle Cambrian fauna. It is thus without 

 very much doubt, the stratigraphic equivalent of the Tonto sandstone of the Grand Canyon 

 section, considered by Walcott as also included within the middle Cambrian, the lower Cam- 

 brian being there represented by the unconformity between the Tonto and the Grand Canyon 

 series. It is possibly the equivalent of at least the lower part of the thicker and more varied 

 Apache group of the Globe district in central Arizona, about 110 miles north-northwest of 

 the Bisbee quadrangle, and also correlated provisionally with the Tonto group. * * * 



The Abrigo Hmestone is named from Abrigo Canyon, 3 miles southwest of Bisbee, where 

 the beds composing the formation are well exposed. Like the Bolsa quartzite, the Abrigo 

 hmestone occurs principally in the northwestern part of the quadrangle, although a few small 

 areas are found within the attenuated belt of Paleozoic rocks extending southeastward past 

 Gold Hill toward Glance Creek. The most satisfactory occurrence, and the only one in the 

 quadrangle where the entire sequence of the beds from base to top may be studied in a 

 single continuous exposure, is that of the Mount Martin section, 1^ miles west of Bisbee. As 

 measured in this section the Abrigo hmestone is 770 feet thick. It rests conformably upon 

 the Bolsa quartzite and is overlain by the Martin limestone (Devonian) . * * * 



The Abrigo hmestone is distinguished from the other calcareous formations of the local Paleo- 

 zoic section by its prevailing thin bedding, and particularly by a conspicuous laminated struc- 

 ture produced by the alternation of thin irregular sheets of chert with layers of gray limestone. 

 The layers of hmestone may be 2 or 3 inches in thickness, while those of chert are usually less. 

 This cherty lamination is eminently characteristic of the Abrigo hmestone in the Mule Moun- 

 tains and serves as a ready means of identifying the beds belonging to this formation within the 

 various fault blocks into which the region is dissected. Chert also occurs occasionally in the Devo- 

 nian and Carboniferous limestones of the Bisbee quadrangle, but as irregular bunches, never in the 

 form of the thin, more or less anastomosing sheets pecuhar to the Cambrian Abrigo hn;estone. 



