300 proceedings: geological society 



ond, that its desiccation lately became complete and remained so to 

 within the last three hmidred years. Russell arrived at the first con- 

 clusion through a study of present conditions and a careful reconnaissance 

 of the rim of the basin. In this he is supported by the ichthyology of the 

 region, as nearly all of the species are indigenous, thus indicating long' 

 isolation. He was forced to adopt the second hypothesis to account for 

 the comparatively fresh condition of the waters of Pyramid Lake. In this 

 he receives no support from a study of the fishes, but on the contrary 

 the facts thus derived point distinctly to the presumption that the pres- 

 ent Pyramid and the ancient Lahontan lakes have been continuous 

 since Quaternary times and that their waters have been constantly 

 fresh enough to support animal and plant life. 



The upper Cretaceous formations of western New Mexico and their 

 relations to the underhjing rocks: D. E. Winchester. In the area sur- 

 rounding the Zuni Mountains in western New Mexico, the Upper 

 Cretaceous is represented by the Mesaverde, Mancos, and Dakota 

 formations, with the latter resting unconformably on older strata. 

 In a section of these formations near the Tres Hermanos Buttes, south- 

 east of the Zuni Mountains, the Dakota is represented by a single 

 conglomeratic sandstone or quartzite bed, varjdng in thickness from 1 

 to 40 feet. The Mancos (1800 feet thick) is more than 60 per cent 

 sandstone and includes four massive, persistent sandstone beds which 

 stand out as prominent cliffs wherever exposed. In its type locality, 

 200 miles to the north, the Mancos is entirely shale. Between these 

 two localities near Gallup, New Mexico, there is a large amount of drab, 

 fissile shale, but the section is still predominantly sandy. The Mesa- 

 verde formation in the Tres Hermanos section contains no persistent 

 massive sandstone, as it does in its type locality 200 miles to the north. 

 In New Mexico both the Mancos and the Mesaverde formations are 

 coal-bearing, while in their type locality in southeastern Colorado, 

 only the Mesaverde contains coal. Lithologically, then, there is a 

 marked change especially in the Mancos between the area about the 

 Zuni Mountains and the type locality to the north, indicating, as has 

 been suggested by W. T. Lee, that the mountains furnishing the sedi- 

 ments were located to the south of both sections. 



The unconformity between the Upper Cretaceous and underlying 

 sediments is very marked. At Zuni, on the west side of the Zuni 

 Mountains, the Dakota rests on top of 475 feet of Zuni sandstone, 

 while at Atarque, 23 miles south, there is only about 75 feet of Zuni 

 sandstone beneath the Dakota. On the east side of the mountains 

 conditions are similar, with about 450 feet of Zuni sandstone beneath 

 the Dakota at Acoma and the sandstone entirely absent near Tres 

 Hermanos Buttes, 22 miles south. If the Zuni sandstone is to be 

 correlated, as apparently it should be, with the La Plata sandstone 

 of southwestern Colorado, pre-Dakota erosion has also removed all, 

 or nearly all, of the McElmo formation (500 to 600 feet) in the area 

 surrounding the Zuni Mountains. 



Frank L. Hess, Secretary. 



