STRUCTURE OF THE TURKS ISLANDS 271 



amidst a labyrinth of sand-hills may on his return traverse a clean- 

 swept plain where not a single medano obstructs his view. Though 

 the sand is not formed of calcareous materials, but is derived from 

 the disintegration of andesitic rocks, the lesson will be the same. 



In February 1904 I spent several days in examining the medanos 

 of the Ancon coast-region north of Callao. Since the details are 

 given in Note 34 of the Appendix, I will confine myself here to a 

 few general remarks. Sand covers the broad plains and the lower 

 hill-slopes and completely hides the crests of hill-spurs, 400 feet in 

 height, as they descend to the coast. At the time of my visit these 

 crescentic mounds, usually six to ten feet high and twenty-five to 

 thirty feet across, formed a line, or rather a column, of two or three 

 irregularly abreast traversing a sandy waste of hill and plain for a 

 distance of from four to five miles, and crossing ridges and spurs 

 300 or 400 feet in height. I watched them as they came into being 

 near the beach and as they died away miles inland a few hundred 

 feet up the slopes of the main range in their fruitless endeavour to 

 scale the mountains. I spent hours in watching them dribbling 

 over the sharp crest of a mountain-spur that descends to the coast 

 immediately south of Ancon. They had reached the crest after a 

 climb of about 350 feet from the beach below, and as I sat on the 

 ridge-top observing them, they peppered my face with their finer 

 6and with each fresh gust of wind. The sand usually formed a con- 

 tinuous slide during the descent of the steeper northern slopes of 

 the ridge, the medanos shaping again when the sand reached the 

 plain ; but where there was a gentler gradient they re-formed half- 

 way down the slopes. During my sojourn the prevailing winds were 

 light, and, as measured by me, the medanos moved forward only a 

 few inches a day ; but before a fresh wind they would advance yards 

 daily. Before a gale they would move rapidly across the plains, and 

 a strong wind blowing athwart their line of advance would in the 

 course of a day or two level them with the ground. Von Tschudi 

 also describes medanos with immovable bases formed around blocks 

 of rocks that are scattered about the plain. While the sand is heaped 

 up by the wind on one side,~~it descends on the other; and there is 

 nothing permanent about this type of medano but its site and its 

 conical shape. The moving medano, however, is the great distributor 

 of sand over the arid wastes of the sea-board of Peru. Sterility 

 reigned over the sand-covered plains and hill-slopes of Ancon. Only 

 occasionally one came upon patches of a little bromeliaceous 

 "tumble-weed" (Tillandsia), which, however, became more frequent 

 as the plains approached the foot of the mountains. 



But, to return to the large islands of the Bahamas, as they origin- 

 ally were, covered with and built up by drifting sand, with little or 

 no vegetation to compete with the dominion of the sand-dune, the 

 conditions of the present are confessedly not those of the past, and 

 in what, we may again ask, has been the change ? I would suggest 

 with some diffidence that the climatic regime which now prevails 

 on the sea-borders of North Chile and Peru once existed in the 

 Bahamian region. Just as the cold waters of the Humboldt or 

 Peruvian Current determine the relative sterility of that continental 



