1922] NATURAL SCIENCE OF PHILADELPHIA 73 



hills, and consequent abrupt ascent from the desert floor, the 

 desert mountain ranges show a common peculiarity of contour 

 which is pictured with especial felicity by J. Smeaton Chase in 

 his very readable book, "California Desert Trails" (:19, p. 7): — 



"The characteristic contour form of the desert mountain ranges 

 is another element in the beauty of desert color. Like geological 

 models set on a table, they stand up sharply defined from the general 

 level, arresting the glance with new, conspicuous effects. No 

 gently modelled approaches prepare the eye for the change of 

 plane. From gray or drab expanse of sand they rear up wall-like 

 profiles of red or ochre. Perspective is dwarfed by the clearness of 

 air, increasing the sense of verticality. Instead of rising from the 

 desert, these mountains stand upon it, explicit, bald, almost arti- 

 ficial." 



Mt. San Jacinto furnishes the supreme example of these sudden 

 changes in level. Measuring horizontally north on the topographic 

 map from the peak to various points in Snow Creek Canyon and 

 vicinity one finds a span of but 3 24 miles from the summit to the 

 2,000-foot contour, or of only 5 miles to the 1,400-foot contour, a net 

 drop of 9,405 feet. To the east of the peak the fall to the 500-foot 

 contour takes place in less than seven horizontal miles. Very fre- 

 quently of course less extreme descents take place much more 

 abruptly. 



Occurrence of Land Snails. — It has been deemed best to 

 include the foregoing brief account of the region because an under- 

 standing of its peculiar topography is of the utmost importance 

 to the malacologist in view of the fact that the land snail fauna of 

 the desert, so far as known, is confined wholly to these steep 

 mountain slopes. From the Colorado Desert in the technically 

 geological sense of Blake, endemic, terrestrial mollusks are pro- 

 bably wholly lacking. The massive granitic fragments which litter 

 the mountain bases, the obliquity, with reference to the rays 

 of the sun, of all slopes but the hot southern exposures, and the 

 modicum of moisture, however scanty, which seeps down from the 

 heights, all serve to render possible the continued existence in 

 such situations of these remarkably adapted xerobic creatures in 

 a way not possible, even for them, on the scorching floor of the 

 desert a stone's toss away. West of Indian Well I found myself 

 literally able to collect Micrarionta xerophila from the steep 

 mountain slope while standing with both feet firmly planted on 



