482 
INLAND DUNES. 
Inland Dunes. 
I liave met witli some observations indicating a structural 
difference between interior and coast dunes, whicli might per¬ 
haps be recognized in the sandstones formed from these two 
species of sand hills respectively. In the great American des¬ 
ert between the Andes and the Pacific, Meyen found sand 
heaps of a perfect falciform shape.* They were from seven 
to fifteen feet high, the chord of their arc measuring from twenty 
to seventy paces. The slope of the convex face is described as 
very small, that of the concave as high as 70 ° or 80 °, and their 
surfaces were rippled. No smaller dunes were observed, nor 
any in the process of formation. The concave side uniformly 
faced the northwest, except toward the centre of the desert, 
where, for a distance of one or two hundred paces, they grad¬ 
ually opened to the west, and then again gradually resumed the 
former position. 
Pdppig ascribes a falciform shape to the movable, a conical 
to the fixed dunes, or medanos , of the same desert. “ The me¬ 
danos,” he observes, a are hillock-like elevations of sand, some 
having a firm, others a loose base. The former [latter], which 
are always crescent shaped, are from ten to twenty feet high, 
and have an acute crest. The inner side is perpendicular, and 
the outer or bow side forms an angle with a steep inclination 
downward. When driven by violent winds, the medanos pass 
rapidly over the plains. The smaller and lighter ones move 
quickly forward, before the larger ; but the latter soon overtake 
and crush them, whilst they are themselves shivered by the 
collision. These medanos assume all sorts of extraordinary 
figures, and sometimes move along the plain in rows forming 
same localities as those examined by Mr. Blake, and the difference in their 
character may denote a difference of origin or of age. 
* Laurent (Memoire sur le Sahara , pp. 11, 12, and elsewhere) speaks 
of a funnel-shaped depression at a high point in the dunes, as a character¬ 
istic feature of the sand hills of the Algerian desert. This seems to be an 
approximation to the crescent form noticed by Meyen and Pdppig in the 
inland dunes of Peru. 
