74 



PERCY (UKNSEL 



fact does not prove that they were not in solution in that material at an earlier 

 time. Hut if one finds the condition siiown by these basalts to be invariably 

 true of all rocks rich in olivine, which have suffered quenching, one must 

 conclude that large amounts of olivine occur in solution in magmatic liquids. 

 A survey of igneous provinces leaves no question that such rocks do have this 

 character, that is, thc\' always contain either all of their olivine or all in excess 

 of a (juite small amount (apparently some 12 — 15 %) as relatively large pheno- 

 crysts. rhe\-, therefore, force acceptance of the stated conclusion" (23, p. 163). 

 The (juotation above refers to purely theoretical questions but indicates the extreme 

 j)osition these rocks hold in petrographic classification with regard to the abnor- 

 mally high content of olivine in basaltic lavas. JoilANNSEN has named these rocks 

 masafuerite, with the following argument: "The picrite basalt from Masafuera of 

 the islands of the Juan Fernandez group is a most extraordinary rock . . . While 

 this particular rock occurs as a dike, on the adjacent island of Masatierra, for example 

 at Puerto h'rances, there is a similar rock in the form of a lava flow with large 

 olivine crystals in a groundmass containing more or less the same mineral. I am 

 placing the rock among the hypabyssals on the basis of the occurrence on Masa- 

 fuera. To all olivine-melabasalt dikes which contain more olivine than any other 

 mineral and in addition carry basic plagioclase and augite, I should like to apply 

 the name masafuerite" (24, p. 334). 



We already have the name picrite basalt for the lava flows of much the same 

 composition on Masatierra, with Lacroix's name oceanite as synonym, given with 

 the following definition: "les roches basaltiques porphyriques a olivine sont parfois 

 extraordinairement riches en peridot; dans I'echantillon analyse, tons les grains 

 de ce mineral se touchent, ils sont reunis par une petite quantite de plagues de 

 labrador, englobant des microlites d'augite et des lames d'ilmenite" (14, p. 44). 

 Johann.sen has restricted the name masafuerite to aschistic dike rocks of a 

 picrite basaltic magma. 



These dikes may represent, at least in part, transmission channels for the 

 upj)er basalt beds, supersaturated with iron oxides. A significant feature in this 

 respect is that they contain numerous 'schlieren' of darker colour, due to abundant 

 minute grains of magnetite. These streaks may indicate relics from a magmatic 

 flow, subse(juently consf)lidated in the shape of the olivine basalt, supersaturated 

 in iron oxides, which now forms the highest parts of the island. At a later period 

 the ciiannels may then have been filled with the melanocratic magma, which now 

 characterizes them as such singular rocks. Sequent intrusions of this nature might 

 lead to phases of crystalHsation in accordance with l^owen's conception of these 

 dikes as (jucnched rocks, referred to above. 



Regional Relations. 

 Tectonic Connections. 



In the introductory lines I already noted that during recent years as well as 

 in older reports speculations have been offered regarding connections in one or 



