Maech 3, 1911] 



SCIENCE 



355 



THE GEOLOGICAL SOCIETY OF WASHINGTOIir 



The 237th meeting of the society was held at 

 the Cosmos Club on Wednesday evening, January 

 11, 1911. 



Regular Program 

 Desert Pavemenls and Analogous Phenomena: E. 



E. Feee. 



Where wind scour acts on unconsolidated desert 

 materials pebble pavements are of common occur- 

 rence. Such occurrences have been described by 

 Blake/ Tolman - and others. As a result oi 

 similar wind scour the surface sand of stable 

 dune areas is often coarser than that underneath. 

 Analogous pavements are occasionally produced 

 by water action. 



Nonnezoshe — the great Natural Bridge of South- 

 ern Utah: Joseph E. Pogue. 

 Southeastern Utah boasts four natural bridges, 

 the Owochomo, the Kachina, the Sipapu and 

 Barohoini (Piute for rainbow) or Nonnezosliie 

 (Navaho for stone arch), each of which surpasses 

 in size the well-known Virginia natural bridge. 

 The first three of these have been called by com- 

 monplace personal names, but the above names 

 are original Indian ones and are far preferable. 

 The largest and most southerly of the four, the 

 Eainbow Bridge, was visited on July 26, 1910, by 

 a U. 8. Geological Survey party consisting of 

 H. E. Gregory, in charge, John Wetherell, K. C. 

 Heald and the writer. This imposing structure is 

 situated in San Juan County, in a wild and well- 

 nigh inaccessible part of the Navaho Reserva- 

 tion, just four miles north of Navaho Mountain 

 and near the junction of the San Juan and Colo- 

 rado rivers. 



The La Plata (Jurassic?) sandstone, here 1,200 

 feet or more in thickness, is deeply dissected by a 

 labyrinth of tortuous canyons, and near the mouth 

 of one of these the bridge is found. A towering 

 arch, rainbow-shaped and of model symmetry, 

 rises from a ledge on one side of the canyon, and 

 spanning a small stream, joins the opposite wall 

 on its downward bend. The opening measures 

 267 feet in height by 278 feet between abutments; 

 but the distance from stream bottom to top of 

 arch totals 309 feet, while the keystone portion 

 is only 42 feet thick by 33 feet wide. The arch 

 is carved from a buff-colored massive phase of the 

 La Plata sandstone, and represents an opening, 

 enlarged and shaped by desert weathering, through 

 which the stream originally cut off one of its 



'Rept. Pae. Ry. Surv., 5: 230, 1S56. 

 'Jour. Geol, 17: 149-151, 1909. 



meanders. The abandoned meander remains as a 

 proof of this origin. 



The bridge was discovered on August 14, 1909, 

 by W. B. Douglas, of the U. S. General Land 

 Office, with four assistants, and Byron Cummings, 

 of the University of Utah, with three students, 

 under the guidance of John Wetherell and two 

 Navaho Indians. It has subsequently been set 

 aside as a national monument and represents the 

 largest and most graceful structure of its kind 

 thus far known. 



Criteria for an Unconformity in the so-called 



Laramie of the Raton Mesa Coal Fields of N&w 



Mexico and Colorado: W. T. Lee. 



During the summer of 1910 the unconformity 

 in the coal-bearing rocks of the Eaton coal field 

 of New Mexico, first announced in 1908 and pub- 

 lished upon the following year, was traced around 

 the Raton coal field in New Mexico and the 

 Trinidad coal field in Colorado, an area extending 

 about ninety miles along the east front of the 

 Rocky Mountains and stretching eastward to a 

 maximum width of fifty miles. The evidences of 

 unconformity may be grouped under two general 

 headings, stratigraphic and paleontologic. 



The formation below the unconformity is coal- 

 bearing and varies in thickness from about 450 

 feet to 0. The formation above the unconformity 

 is likewise coal-bearing and is marked by a con- 

 stant basal zone of conglomeratic sandstone. The 

 relation of the basal conglomerate of the upper 

 formation to the beds below leaves little room for 

 doubt that the variation in thickness of the lower 

 formation is due to erosion. In at least four 

 places the lower coal-bearing formation is want- 

 ing and the basal conglomerate of the upper one 

 rests upon older rocks. This basal conglomerate 

 contains pebbles of coal which must have come 

 from the lower coal formation, pebbles of con- 

 glomerate which could come only from the Dakota, 

 stratigraphically about 3,500 feet below, or from 

 some formation still older; pebbles of red sand- 

 stone which could come only from the red beds, 

 the top of which is about 4,000 feet below ; pebbles 

 of horn corals and fossiliferous cherts, such as 

 are now found in the Carboniferous rocks west of 

 the coal fields, about 18,000 feet below; and a 

 variety of metamorphic and igneous rocks, in- 

 cluding crystals of feldspar supposed to come from 

 the crystalline complex of the mountains. Ap- 

 parently these pebbles prove that after the earlier 

 coal measures were formed the mountains west 

 of the Raton Mesa region were elevated and the 

 upturned stratified rocks, having a measured 



