I02 C. SKOTTSHKRG 



More normal olivine basalts, less rich in olivine than the rocks character- 

 istic of the lower horizons, are "widespread up to the highest part of the island" 

 (Oiiensel 2 p. 49). They are dark gray and as a rule vesicular, scoriaceous or 

 slaggy, but they are hard, more resistant to denudation, and form the elevated 

 crests and crags all along the ridges. At intermediate horizons, approximately 

 between 400 and 500 m (Cordon Chifladores 400 m, Portezuelo 500 m), feldspar 

 basalts seem to predominate (Ouensel I.e.). These lavas are ash gray, aphanitic and 

 aphyric in texture and have a tendency to develop a columnar structure. Rocks 

 of the same type were found near Pta Larga at less than 100 m in the form of 

 a dike, "which may signify a channel for the analogous lavas at higher levels". 

 These beds are supposed to "represent a definite epoch of intrusion, intermediate 

 between the doleritic basalts and picrite basalts of the lower parts and the sco- 

 riaceous olivine basalts of the higher horizons". Very likely the thick bed seen 

 on PI. 97:2 in Skottsb. 3, about 450 m above sea level, belongs to this type. 

 I admit that the photograph selected by Quensel to illustrate this formation (fig. 

 9, p. 51) has the same outward appearance, but the altitude is approximately 

 625 m, and as this place is out of reach — the climber cannot depend on the 

 shallow-rooted shrubs and ferns — no specimens were taken there; the samples I 

 brought came from 500 to 575 m, and whether or not the beds are intermediate 

 is impossible to tell, as no rocks from a higher elevation than 575 m have been 

 examined. The samples from this level are vesicular aphyric feldspar basalts. We 

 know nothing about the mineraiogical composition of the rocks forming the 

 highest summit. Mt. Yunque rises about 350 m above Portezuelo de Villagra. 



A geographical reconnaissance of Masatierra. 



Pta Ilueso de Ballena, where we shall start our circuit of the island, plunges 

 abruptly into the sea, forming an escarpment of perhaps 300 m. A dominant 

 feature of the coast is that talus deposits are insignificant or lacking, so that 

 the surf is able to undermine the wall and to excavate caves. Only in the coves 

 where a valley has been eroded down to sea level, a beach is found which leaves 

 a f)assage along the foot of the escarpment. 



Between the east cape and the Frances valley the land rises to 500 m or 

 more. Some shallow cjuebradas, filled with forest, descend north toward the sea 

 but do not reach very far down (fig. 11). 



Pto I-nviccs does not deserve to be called a harbour; it offers no protection 

 even as an occasional anchorage (Instrucc. naut. p. 226). It is a small, open cove 

 facing N and IC, but with winds from other quarters landing is easy. The beach 

 consists of rounded stones and coarse shingles; here as elsewhere the surf re- 

 moves all minor particles. Tlie lower slopes of the valley are very barren, the soil 

 is exposed or covered with })atches of weeds, and the marks of running water 

 and the tracks of cattle are everywhere to be seen (fig. 12). Some little distance 

 from the shore and about 50 m above sea level is a small shack. The streambed 

 occupies the entire narrow bottom of the broadly V-shaped valley to which sev- 



