A GKOGRAHHICAI. SKETCH OF THE JUAN P'ERNANDEZ ISLANDS 



99 



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Fig. 7. Diagrammatic sketch of a profile of the seaward base of Mt. Vunque. Height c. 350 — 400 m. 



3-sided prisms. The dolerites have a very fresh appearance and are supposed to 

 represent intrusions between previously consoHdated flows (Ouensel 2 pp. 45, 47). 



The lava beds at lower elevations in east and central Masatierra are inter- 

 bedded with agglomerate layers, formed by tuffaceous material as explained by 

 Quensel and illustrated by his fig. 1 1 (2 p. 54). The photograph was taken near 

 the entrance to Anson's valley at approximately 50 m above sea level. In a 

 clayey ground mass of a deep brick-red colour hard blocks of various shapes 

 and sizes are embedded, showing spheroidal weathering; I.e. fig. 10 is a fine 

 exainple froin the same bed. Another, from the floor of Valle Colonial near the 

 trail to Portezuelo, is seen in my fig. 6. The entire exposed surface weathers in 

 this fashion; the soft ground mass is washed out and carried into the sea and 

 the hard blocks left lying. The same kind of agglomerate is found also in other 

 valleys. Stratification is quite distinct in the profile illustrated by Quensel. His 

 opinion is that these beds are pyroclastic sediments of recent volcanic origin. 

 According to my notes the volcanic agglomerate was observed overlayered by hard, 

 gray basalt in Pto Frances and at the seaward base of Mt. Vunque (fig. 7). In 

 the profoundly eroded Cuinberland valleys all the upper strata have been removed. 



All through the island the lower horizons are traversed by vertical dikes of 

 hard lava striking approximately N — S. Nowhere are they better observed than 

 on the imposing perpendicular escarpment between Vaqueria and Juanango 

 bay, where the wall is ribbed with innumerable dikes which, thanks to their greater 

 hardness, project above the rim to form a serrated edge (figs. 8, 9). 



The geology of Bahia del Padre was considered by PoEHLMAXN to be of 

 particular interest and importance. The lowermost bed at the entrance to the bay 

 was identified by him as an andesite representing a inuch older formation than 

 the overlaying basalts and tuffs and exposed only in this place, but Ouensel came 

 to the conclusion that we have no reason to classify these beds as andesites of 

 an older formation (i p. 266, 2 p. 56); they "have been subjected to alterations 

 in connection with thermal processes during some intermediate phase of volcanic 

 activity". 



