5 6 PERCY QUENSEL 



however, very much altered to serpentine, chlorite and prehnite. Secondary albite 

 and some sca[)olite is also present together with abundant p\Tite. Small cavities 

 are filled with epidote and calcite. In the slaggy lavas in the immediate vicinity 

 larger cavities are filled with a purely white clay like substance which was found 

 to be newtonitc. In intimate association with this mineral is nearly always found 

 a yellowish waxy substance which shows every resemblance to what mineralogi- 

 cally may be signified as bole (Fig. 13). It is very brittle with a conchoidal 

 fracture. In water it readily disintegrates into small angular fragments. This bole 

 mineral is also found filling cracks, or occurs in smaller masses in the cavities 

 of the basalts around the bay. 



In the agglomeritic lavas of the vicinit}- large cavities are filled with hard 

 compact magnesite. As early as 1886 Darapsky described this mineral b\' the 

 name "Glockenstein" and gave an analysis thereof, which shows it to be an 

 exceptionally pure magnesite (9, p. 113). Without doubt the magnesite is primarily 

 derived from olivine, the decomposition being caused by the same processes as 

 have changed the basaltic lavas nearb}-. 



The whole aspect of the rocks from Bahia del Padre, with magnesite, calcite, 

 serpentine, chlorite, scapolite and jn'rite as secondary minerals, seems without 

 doubt to indicate that the lava beds in question have been subjected to alterations 

 in connection with thermal processes during some intermediate phase of volcanic 

 activity on the island. There seems no reason to classify them as andesites of 

 an older formation, as is done by Schulze and Pohlmann. 



In a specimen collected by Skottsberg from the shore south of El Yunque, 

 Hagerman also found evidence of a far-reaching decomposition. Under the heading 

 "Hydrothermale Bildungen" he gives the following description: "Das Handstiick 

 ist ein von weissen Streifen durchzogenes scharfgriines Gestein, das u. d. M. 

 grosse Augitkristalle in einer vollig zerflossenen Serpentinmasse zeigt. Das Pra- 

 parat ist von Aragonitbandern durchzogen. Dieses Gestein muss als ein stark 

 umgebildetes Olivinfels bezeichnet werden" (13, p. 27). The large olivine crystals of 

 the dunite, found as inclusions in the picrite basalt at Puerto Frances, show, on the 

 contrary, no signs of secondary alteration (see p. 47). There is therefore no doubt 

 that the highly decomposed 'Olivinfels' described by Hagerman has succuinbed 

 to a later decomposition of much the same nature as has been active around 

 Bahia del Padre. 



Masafuera. 



(Lat. 33° 52' S., Long. 80° 54' W.) 



The lavas of ^^lasafuera present a more var}'ing aspect than those of Masa- 

 tierra and contain several types of petrographic interest. They have, however, 

 hitherto only been summaril\' described in the {)apers by mv'self (12, p. 274) and 

 by Hagerman (13, p. 28). 



The rocks which predominate at lower levels are mostly vesicular to slaggy 

 basalts. They are well represented around the Ouebrada de las Casas. At higher 

 levels the basalts consist of more compact lava beds. In contrast to the basalts 



