82 BULLETIN 56, UNITED STATES NATIONAL MUSEUM. 



Buckley) occupy areas in the direction of the Palomas Lakes. 

 Ephedras arc the most abundant shrubs. 



The Seca and Aguila mountains, though barren, are rich in color- 

 ing- aiid singular in form. Two elevations, about 3 kilometers (2 

 miles) southwest of Monument No. 15, were capped by nipple-like 

 buttes of red hornblende-mica-andesite. Below the summits were 

 narrow, horizontal, blackish strata of the same rock, the red variety 

 again appearing below and forming the bulk of the rocky peaks, 

 which rest on a massive base of glassy rhyolite containing opal and 

 an abundance of andesite. A lower ridge a little east of south from 

 the monument and about 1.0 kilometers (1 mile) distant from it is 

 capped with a black, impure obsidian in large irregular masses. Un- 

 derlying the obsidian is a liver-colored, line-grained, homogeneous 

 rhyolite in contact with a red. glassy form of the same in a broad 

 stratum under it. Below these is a slate-colored rhyolite, in the form 

 of plates, which ring like metal. Still farther north, nearly on the 

 mesa level, is an extensive cropping of quite pure vitreous obsidian. 

 Between the two localities above described is an enormous dike of 

 hornblende-mica-andesite, varying in color from steel gray to reddish 

 brown. Near and to the south of it is a peak formed by a most 

 hetrogeneous mixture of volcanic rocks on an enormous scale and of 

 extremely coarse structure. The cavities in the amygdaloid rock of 

 this region discharge chalcedony, rock crystal, agate, etc., which lie 

 thickly scattered on the slopes at the bases of some of the cliffs. 

 Neighboring areas are covered with basaltic lava or fragments of 

 black obsidian and blackish rhyolite. Geologic conditions were here 

 found to have produced a distinct effect upon the local distribution 

 and coloration of animals. The sandy soil varies in color from 

 grayish white to yellow and red, and this variation is correlated with 

 similar variations in the coloring of the reptiles and mammals of the 

 region. The snakes, horned toads, and other lizards evinced a 

 decided susceptibility to the influence of the color tints of their sur- 

 roundings in the localities above described, and it was possible to dis- 

 tinguish the rabbits living in beds of chalcedon}' mixed with whitish 

 sand from those inhabiting the dark lava flows and beds of obsidian 

 chips. 



Station No. 8. — Palomas Lakes, Mimbres Valley. Chihuahua, 

 Mexico. One mile south of Monument No. 21. Altitude. 1.210 

 meters (3,970 feet). The Palomas Lakes occupy the lower portion 

 of the Mimbres Valley, forming a chain that ends in Lake Guzman, 

 the lowest part of the basin, in Chihuahua. Mexico. Lake Guzman 

 is also fed by the Corralitos River, which enters it from the south, 

 as well as by other smaller streams. Collections, especially of fishes, 

 were made in Lake Guzman by the naturalists of the old survey; 



