REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 115 



These dolerites are rarely free from alteration ; microscopic sections show that they are 

 almost always penetrated by delessite, which even invades the crystals of plagioclase, 

 and they are further often covered with hydrate of iron ; grains of red hematite, also, 

 are often seen. As we said when speaking of the macroscopic characters of these rocks, 

 they are often amygdaloidal and filled with zeolites ; chabasite is the most important 

 of these, either completely filling the vesicles or lining their walls. 



Fine-grained felspathic basalts were also collected at Christmas Harbour. The 

 specimens examined were taken from a bed above sea-level in the northern part of 

 this locality. Viewed by the naked eye these rocks are black, very compact, breaking 

 with a plane fracture, and sometimes presenting large crystals of felspar and olivine. 

 In some cases the rocks are altered, and take a greyish tint ; the olivine decomposes 

 into a greenish substance like steatite, and the felspar into kaolin. These altered rocks 

 are often clothed with a thick coating of fibrous zeolite. Under the microscope these 

 rocks are seen to be felspathic basalts ; olivine is the only microporphyritic constituent. 

 The larger sections of this mineral are transformed internally into a fibrous greenish 

 dichroic matter, which is perhaps chlorite, possibly even a mica ; a brownish frame 

 surrounds the olivine crystals. The ground-mass, in which quadratic sections of 

 magnetite abound, is formed of small grains of augite 

 and opalised felspar microliths. The microscopic vesicles 

 are bordered with fibro-radial delessite, the centre being 

 filled with analcime, and in certain cases by a fibro-radial 

 zeolite. 



The olivine of a fine-grained basalt from the same 

 place, and closely resembling that just described, presents 

 interesting peculiarities. It appears in grouped granules, 

 imitating to some extent the peridotic chondres of 

 meteoric rocks. Fig. 19 represents these groupings of F '?- ™-" ° f "^ Harb ™ r - 



° ■■■ o j. o Grouped granules of olivine, lmitat- 



olivine grains, which are numerous enough in this rock ing the form of this mineral in the 



o o chondres or meteorites, ^u crossed 



to form a characteristic feature. mcols - 



Another basaltic rock, from a bed 400 feet above the coast, shows some noteworthy 

 peculiarities. The ground-mass is black and compact ; large crystals of felspar and 

 olivine appear in it, and the fracture is irregular. Microscopic examination shows that 

 it is a felspathic basalt like those already described, but while in the former case it was 

 olivine which gave these rocks a microporphyritic structure, here large sections of plagio- 

 clase produce this feature. They stand out from a ground-mass of grains of augite, 

 felspathic microliths, and granules of olivine. These large crystals of plagioclase present 

 a character sometimes shown by anorthite and certain albites ; their sections appeal- 

 almost free from hemitropic striae. It is well known that the felspars which form the 

 beginning and the end of the plagioclastic series have generally less numerous stria? 



