118 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



few inches to lo feet, but are usually thinner than those of the Escabrosa 

 [Mississippian] limestone. [The Naco limestone isj compact and nearly aphan- 

 itic, ringing under the hammer, and breaking with a splintery fracture, whereas 

 the Escabrosa limestone is usually more granular and crystalline, and crumbles 

 more readily when struck. There are, however, exceptions to this rule, dense 

 aphanitic beds occurring rarely in the Escabrosa formation and granular crinoidal 

 beds being not uncommon in the Naco limestone. * * * 



"While the greater part of the 3,000 feet or more of the Naco formation is 

 made up of fairly pure gray limestone, certain thin beds of a faint-pink tint occur 

 at several horizons * * *. These pink beds, which weathering usually shows 

 to have an inherent lamellar or shaly structure, are very fine grained and compact 

 in texture. They effervesce freely with cold dilute acid and are evidently com- 

 posed chiefly of calcium carbonate. Examination of natural surfaces with a lens, 

 however, shows the presence of minute quartz grains and tiny flakes of mica. * * * 



"Chert is not uncommon in the Naco formation; it occurs sometimes as 

 irregular bunches and nodules in beds of otherwise pure limestone and sometimes 

 as the result of silicification of thin fossiliferous beds throughout their thickness. 

 It is also particularly abundant along and near zones of Assuring and faulting. 



^^ Conditions of deposition. — As littoral sediments are entirely lacking and 

 terrigenous materials represented only by the very minute mica scales and quartz 

 grains in the pink calcareous shale, it may fairly be concluded that the Naco 

 limestone, which is not noticeably dolomitic, was deposited in moderately deep 

 water at some distance from the shore, whence the tiny mica scales were derived. 

 * * * The region of deposition was in the main beyond the 'mud line,' which the 

 results of the Challenger expedition showed to He generally at a depth of 100 

 fathoms. During certain stages of the accumulations of the limestones, offshore 

 currents carried some of the finest of the land waste into this area of tranquil 

 deposition and left records of these occasional incursions in the form of pink 

 shales." 



Girty says,^ after listing the fauna of the Naco: 



"Very few of these species have exact representatives in the Mississippi 

 Valley Pennsylvanian * * *. Productus ivesi, Productus occidentalis, and Archeo- 

 cidaris ornata are suggestive of the Aubrey limestone of the Grand Canyon 

 section, just as the abundance of Omphalotrochus suggests the ' Permo-Carbon- 

 iferous' of California. The whole fauna is closely related to that of the limestones 

 of the Hueco Mountains in western Texas. * * * Its age seems to be late in the 

 Carboniferous, perhaps about the same as the series just referred to. The fauna 

 has at the same time a very different facies from that of the Guadalupe fauna, 

 as well as from that of the so-called 'Permo- Carboniferous' of the Mississippi 

 Valley." 



The top of the Naco is cut off by a great unconformity. 

 The Globe district was described by Ransome^ in 1903. Of the late 

 Paleozoic deposits he says (page 40) : 



"Wherever thick sections of the Globe limestone are exposed it is found that 

 the alternating buff and gray limestones with subordinate grits are overlain by 



^ In U. S. Geological Survey, Professional Paper No. 21, page 54. 



2 Ransome, F. L., Geology of the Globe Copper District, U. S. Geological Survey, Profes- 

 sional Paper No. 12, pp. 40, 109, 1903. 



