180 THE VOYAGE OF H.M.S. CHALLENGER. 



looks slightly grey when seen by ordinary light, remains obscured between crossed 

 nicols. The augite is seen in roughly formed grains embedded between the felspathic 

 sections. The olivine, of which large sections are seen, is uniformly changed into red 

 hematite ; these sections, however, still show extinctions like those of unaltered olivine. 

 In this rock, as in all the other rocks of Juan Fernandez, magnetite is often observed 

 in elongated sections. In addition to the products of decomposition of plagioclase and 

 olivine, small patches of olivine are to be seen. Some other specimens collected on 

 the shore differ neither in structure nor in mineralogical composition from those just 

 described. It is consequently to be inferred that Juan Fernandez is principally 

 composed of basaltic rocks. 



