362 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



and break down into very fine red sand. TossHs are scarce but may be found here and there 

 in sufficient quantity and distinctness to identify their age. These fossils, so far as I have 

 seen, are the saine as those which abound in the beds above therh. 



The upper Aubrey is conaposed largely of sandstones, but they have a very different aspect 

 from those below. In color they are yellowish brown and the cement, instead of being calca- 

 reous, is siliceous — in fact, a regular chert. * * * These sandstones are conspicuously 

 cross-bedded, and the silicification of the rock has in no way obscured it. * * * There are 

 several bands of these adamantine sandstones, and intercalated with them are three or four 

 thick beds of pure limestone, containing an abundance of fossils of many and characteristic 

 species. 



While the Mesozoic strata of this district show some differences in comparison with those 

 north of the Grand Canyon, the upper Aubrey shows none, except in its thickness. Its cliffs 

 in the upper wall of the Grand Canyon and those overlooking the granite of the Zuni Plateau 

 are so similar that it is hardly possible to doubt that their' materials came from the same source 

 and were deposited under the same conditions. In the former locahty they are thicker in about 

 the ratio of 5 to 3, and this ratio holds good not only for the Aubrey series as a whole but for 

 all its subdivisions and principal members. In the Grand Canyon the thickness of the Aubrey , 

 is about 2,000 feet; here it is about 1,200. 



I 13. NEW MEXICO. 



Lindgren ^^'^ describes the Carboniferous of New Mexico as follows : 



The Mississippian, or lower Carboniferous, has been recognized at several places south of 

 latitude 34°. W. T. Lee found limestone of this age in the Ladrones Range and Gordon 

 believes, on the basis of evidence collected by C. L. Herrick, that the lower part of the section 

 in the Magdalena Mountains belongs to this series. Characteristic Mississippian faunas were 

 found by Gordon at Kingston and Hillsboro, and the horizon has for some time been known to 

 be represented at Lake Valley, where a thickness of over 200 feet of limestone has been meas- 

 ured. Rocks of the same age are also present in the Silver City district. Gordon states that 

 at Hillsboro these limestones rest upon the eroded surface of the Devonian calcareous shales, 

 but farther west there is no evidence of unconformity. , 



The Pennsylvanian, or upper Carboniferous, is deposited with a considerable thickness 

 over the whole Territory and reaches its maximum in the country between Santa Fe and Las 

 Vegas. As far south as the latitude of Socorro the Pennsylvanian consists in large part of 

 sandstones and shales in repeated alternation with some limestone beds. But south of this 

 line the pure limestones prevail and at the same time the total thickness appears to diminish. 

 Everything indicates near-shore conditions in the northern part of the Territory, where some 

 land areas probably existed even at that time. 



In northern and central New Mexico there appear to be two divisions of the Pennsylvanian, 

 the upper in certain parts possibly including the Permian. According to W. T. Lee and C. H. 

 Gordon "^ the Pennsylvanian of central New Mexico is divisible into two groups, the lower called 

 the Magdalena group and the upper called the Manzano group. The Manzano consists in fact 

 of the "Red Beds" of Carboniferous age. 



The average thickness of the Magdalena group in the vicinity of Socorro is 1,500 feet. 

 This group is subdivided into (1) a lower formation called the Sandia, consisting chiefly of 

 shales, limestones, and sandstones, the first two predominating, with a thickness ranging from 

 500 to 700 feet; and (2) an upper formation called the Madera limestone, 300 to 500 feet thick, 

 consisting of blue limestone with some shale. Both formations are weU exposed in Socorro and 

 BernalUlo counties. 



The Manzano group, best exposed on the east side of the Rio Grande near Socorro, consists 

 of 2,000 feet of red and variegated sandstones, shales, limestones, and gypsiferous beds. Its 



"Gordon, C. H., Note on the Pennsylvanian formations in the Rio Grande valley: Jour. Geology, vol. 15, 1907, 

 pp. 805-816. Lee, W. T., and Girty, G. H., The Manzano group of the Rio Grande valley, New Mexico: Bull. U. S. 

 Geol. Survey No. 389, 1909. 



