A GEOGRAPHICAL SKF.TCH OF THE JUAN FERNANDEZ ISLANDS 



97 



Fig. 4. Masatierra and Santa Clara right, behind the promontory seen in S\V from 3000 m alt. 



— Photo B. Frodin ^'^ 1952. 



were described by Hagerman. The joint material served (Juensei for a renewed 

 study, enlarged to a discussion of the geotectonic connection between Juan Fer- 

 nandez and other volcanic islands of the East Pacific. 



Even a casual visitor cannot fail to observe the ditterence in appearance 

 and colour between the lower brownish, yellowish and reddish slopes and profiles 

 and the higher, light to dark gray ridges; see the water colour sketch in Skottsb. 

 2, opposite p. 52. These two horizons can be followed from the east end to Tres 

 Puntas, possibly to Cerro Enrique, but no samples were brought from the ex- 

 treme western section with the exception of a tew from Bahia del Padre. The 

 island is "in the main formed by a rather uniform series of basaltic lava beds, 

 only diverging in respect of coarser and finer grain or of a higher or lower 

 content of olivine" lOuensel 2 p. 44'. Rocks with a very high content of olivine 

 (picrite basalts) seem to be restricted to lower elevations: higher up more nor- 

 mal basalts, less rich in olivine, predominate, but between the two extremes there 

 is every transition. Of the lower lava beds, up to 200 m above sea level, "many 

 show a coarse-grained ophitic texture and may be classed as dolerites" (\.c. p. 45); 

 these lavas have been traced from Pto Frances to Tres Puntas. The dolerites are 

 very resistant and show, at least where observed by me, a columiUar structure. 

 They form thresholds in some of the valleys. The most conspicuous ones were 

 met with in Vaquena (fig. 51 — a piece of a column was figured by Hagerman 

 p. 28 — and on the south side of the island below Chumacera and Tres Puntas. 

 At Chumacera the bed is about 3 m thick. The pillars appear as long and narrow, 



