g6 C. SKOTTSBERG 



eluding the Yunque massif, the crest follows the south coast, an imposing bar- 

 ranca several hundred m high, with almost vertical gorges. As a rule the saddles 

 are impassable (figs, ii, 14, 31). From the saddle between Yunque and Piramide 

 (figs. 21, 22) the ridge turns NNW and in a shallow curve runs right across the 

 widest part of the island. The reason is that the beds, at least the middle and 

 upper ones, are horizontal; at the foot of Mt. Yunque a very insignificant tilt of 

 5 — 9° was observed in one place (fig. 7). The central part of the island receives 

 the greatest precipitation, and erosion has worked inland from two opposite direc- 

 tions, but the Cumberland valley system has been considerably more deepened 

 than the Villagra system. The ridge is 600 to 700 m high in this section. A 

 narrow pass, Portezuelo de Villagra, often spoken of as "Selkirk's lookout", forms 

 the only practicable passage between the two sides of the island. West of Vaqueri'a 

 the ridge reaches the north side of the island, turns SW and follows the coast, 

 rising to at least 500 m in the highest peaks, then getting lower and lower and 

 disappearing as we approach El Puente, fiat and sandy and only 50 m above 

 sea level. The small peninsula forming the extreme west of Masatierra is crowned 

 by a hill at least twice as high. At Carvajal the beds appear to be horizontal 

 (fig. 47), but E of the isthmus they are tilted SE, and consequently all the valleys 

 trend toward the south coast. The dip is slight. 



The change in position of the backbone, combined with its decreasing eleva- 

 tion, has a profound influence not only on the morphology, but, as a consequence 

 of the direction of the prevailing winds, also on the local climate and thereby on the 

 vegetation. Climatic dates will be found in my paper on the vegetation (3 pp. 812 — 

 818); the common wind direction is SE to SW (together 78 %). Along the east 

 and central section the air currents are suddenly forced up over crests 500 — 

 900 m high, cooled and condensed, and rain drenches the ridges (fig. 4). This is the 

 forest country, where the deep valleys are covered with verdure. The region 

 around Mt. Yunque may be shrouded in mist while all the country west enjoys 

 sunshine. Very often the lower cloud limit is knife-sharp (Skottsb. 3 fig. 2 on 

 p. 808). Fig. 4 shows clouds also over the West and on Santa Clara. But W of 

 Cerro Chumacera, where the main ridge forms the upper edge of the long north- 

 ern escarpment, the air does not hit a high, precipitous wall but rises gradually, 

 and the elevation is too modest to allow the rain-bringing clouds to gather ex- 

 cept now and then during the winter months. This is the barren, treeless, grass- 

 covered land. 



Geology and morphology. 



No extensive geological survey has been made in these islands. Our know- 

 ledge is mainly based on Quexsel's short visit in 1908, when he studied the 

 stratigraphy at a limited number of places and later gave an account of the 

 geology, petrography and mineralogical composition of the rocks, but his mater- 

 ial was too small to allow us to trace the different kinds of strata from one end 

 of the island to the other. For my own part I had no geological training, but 

 during our 1916 — 17 campaign I collected rock specimens in many places. They 



