162 On the Campus 



some peculiarities. They are gypsum, as everybody 

 knows, 1 but while they move as other sands, they must 

 be compared with wet sands. The vast drifts, thirty to 

 fifty feet in height, are moist often to within a few inches 

 of the surface, and are so compactly driven that one may 

 walk upon the solid surface with comparative ease. A 

 white wall like to the appearance of marble is moving 

 slowly eastward, whelming all vegetation as it goes, some 

 of which, able to grow through the encroaching mass, 

 persists so that all the plants now appearing on the sur- 

 face, so far as examined, are anchored by lengthened 

 stems or roots to the underlying older soil. The same 

 yucca that appears at Escondida here emerges sometimes 

 by green tips from a snow-white drift twenty feet in 

 height, or anon, seems to crown triumphantly some 

 lower mound. The mesquite holds on, in some places a 

 desperate fight, and certain species of Rhus, R. aromat- 

 ica and R. trilobata, perhaps, maintain a perilous exis- 

 tence out over the whole region, sometimes even on the 

 summits of the highest knolls. These sumacs are the 

 characteristic species of the white sands. 



But let us turn north. A journey of fifteen or twenty 

 miles brings us to the black wall of the lava flow. This 



i The following analysis of this material has been kindly fur- 

 nished me by Dr. L. S. Andrews, late of the Mallinekrodt Chem- 

 ical Works, St. Louis: 



Calcium sulphate, CaSO 4 77. 64 per cent. 



Water, H 2 20.55 



Calcium carbonate, CaCO 3 . 95 ' ' 



Silica and undetermined, SiO 2 , etc . 86 ' ' 



100.00 " 



