THE ZUNI SALT LAKE 189 



the rim. It attains a thickness of over 100 feet on the north and 

 southwest portions of the rim, where it rises in a ridge of considerable 

 prominence. The material is mostly stratified, fine-grained, and in 

 part cross-bedded. It consists mainly of scoria, but includes frag- 

 ments of various kinds of sedimentary rocks, including Carbon- 

 iferous (Aubrey) limestones with characteristic fossils. 1 



The view from the rim of the depression is impressive. The ground 

 sinks for 150 feet or more in a circular area a mile in diameter, with 

 flat bottom in part occupied by the lake and in part by the glistening 

 salt and mud flats. The two volcanic cones in the center of the basin 

 are prominent features, rising steeply to an elevation of nearly 150 

 feet above the lake. The larger cone has a deep crater in its summit, 

 which contains a circular pool, about 1 50 feet in diameter, at the lake 

 level. The water of this pool is very salt, but several per cent, less so 

 than that in the main lake. The cone is nearly circular, except on its 

 western side, where it merges into the remains of a slightly older vol- 

 canic mass. The second cone is a short distance northeast, rising 

 steeply out of the mud flat at the southeastern margin of the lake. It 

 has no crater, is smaller, and appears to be somewhat older than the 

 other cone. Both cones consist of scoria and other volcanic ejecta, 

 and they are similar to the cones in the numerous lava fields of New 

 Mexico. Walls of Cretaceous sandstone encircle the depression, 

 which to the north, east, and southeast are capped by a sheet of lava 

 from 30 to 50 feet thick. To the west and southwest the sandstone 

 wall rises in cliffs and rocky slopes about 150 feet above the bottom of 

 the depression, and is capped by rolling hills composed of beds of 

 fragmental volcanic materials, while to the north, east, and south it 

 rises from 60 to 70 feet to the base of the lava sheet. The lava sheet 

 which caps the sandstone walls on the southeast, north, and east 

 sides of the depression is an ordinary sheet of "malpais" or black 

 lava, which appears to be older than the cinder cone with the crater 

 in it. 



1 These fossils were determined by Dr. George H. Girty as follows: 



Lophophyllum sp. Phillipsia sp. 



Productus ivesi. Chonetes n. sp. 



Prbductus mexicanus ? Productus occidentalis. 



Productus sp. Seminula mexicana. 



Bakewellia ? sp. Schizodus sp. 



Laeviadentalium cana. Euomphalus sp. 



