ADDITIONAL COMMENTS ON THE OEOLOGY OF THE JUAN FERNANDEZ ISLANDS 53 



as representing recent tuffacious material. The one he describes as "ein poroses, 

 dichtes Gestein von El Puente, dem Istmus zwischen der Padrebucht und Carbajal 

 und ist ein Palagonittuff mit einigen sporadischen Augit- und Magnetitkornern. 

 Der andere Tuff stammt von dem nordlichen Ufer der Padrebucht, von wo einige 

 umgewandelte Olivinbasalte herriihren" (13, p. 26). As these specimens have 

 been collected without any observations regarding their petrological position, one 

 cannot draw any conclusive evidence as to their age, but the specimens from both 

 the localities have every appearance of being recent pyroclastic sediments. 



I have previously described rocks from Bahia Cumberland, filling out the 

 greater part of the bay, which I assumed to be of recent tuffaccous origin. The 

 description was given as follows: "In den zentralen Teilen von Cumberland Hay 

 liegen noch ziemlich machtige, meist lebhaft rot gefarbte Tufflager, die sehr stark 

 umgewandelt sind. Bruchstucke von Olivinkristallen, lapilliartige Lavabruchstucke, 

 P>zkorner und Glas liegen in einer Grundmasse, die aus einer weichen, mit Mes.ser 

 schneidbaren, roten lateritahnlichen Lehmsubstanz besteht, die durch Verwitterung 

 aus dem ursprlinglichen Tuffmaterial hervorgegangen sein durfte. Uberall in den 

 Tuffsedimenten ist eine deutliche Lagerung sichtbar" (12, p. 266). It is over 40 

 years since I visited the locality and naturally I cannot now rely on any personal 

 recollection. The inundation in connection with the volcanic disturbance of 1835, 

 referred to above (p. 41), may have wrought such havoc, that superficial deposits 

 could have been re-formed. But the composition of the formation, as well as my 

 notes from the field, offer indications that the tuffaceous material of Bahia Cumber- 

 land also represents pyroclastic sediments of recent volcanic origin. 



Even if only trivial remains of tuffaceous formations, indicative of late volcanic 

 activity on the island, are left, the submarine explosion of 1835 confirms without 

 doubt that the area in the immediate vicinity has at that time been subjected 

 to disturbances of volcanic nature. 



Some rocks from around Bahia del Padre deserve special notice. Schulze 

 and Pohlmann have observed the deviating nature of the rock assemblage and 

 the latter has commented thereon as follows: "De suma importancia para esplicar 

 la formacion jeolojica de Masatierra es la entrada a Bahia del Padre, situada en 

 la parte suroeste de la isla. Aqui se observa debajo de las capas basalticas ya 

 descritas un grand macizo de roca compacta verdosa, que es andesitica. Segun 

 la opinion de Schulze' que, a me parecer, es correcta, esta roca verdosa es la 

 mas antigua de la isla. P^n ningun otro punto, ni en Masatierra, ni en Masafuera 

 se ha observado una configuracion jeolojica analoga a la mencionada" (11, p. 4). 



The rocks referred to by Pohlmann certainly show a divergent aspect, l^ut 

 I do not think they can be taken as represehting rocks belonging to a more 

 ancient formation than the lavas around, nor that they should be classified as 

 andesites, as Pohlmann has assumed. I have as the result of microscopical 

 examination of the rocks come to the conclusion that the deviating character is 

 the result of post-volcanic alterations through thermal processes. In my earlier 

 paper I have given in some detail the reasons for this conclusion (12, p. 266), and 



' Ur. Schulze died before publishing his observations. 



4 — 516795 The Nat. Hist, of Juan Fernandez and Easter Isl. Vol. I. 



