REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 27 



augite or epidote. Microscopical examination shows that this rock may be considered 

 as forming a transition between the series of diorites and of diabases. Although much 

 altered, we can class it amongst the Diabas Mandel stein of the German lithologists. 

 The ground-mass is formed almost entirely of felspar associated with numerous grains 

 of epidote and other decomposition products, while a glimpse is sometimes caught of 

 small vague prisms of augite. Crystals of felspar of the first generation are rather well 

 developed ; they occur as thick shortened prisms, several being sometimes grouped 

 together. They are twinned according to the albite law. The angles of symmetrical 

 extinction on the two sides of the hemitropic lamellae, and of that following the trace 

 of M, are generally somewhat small, seldom exceeding the average value of 7° or 8°. 

 Augite is the best represented mineral of first generation ; it appears as grains, and 

 shows the characteristics w T e recognise in amphibolic rocks such as diorite. There 

 would be no hesitation in classing this rock as a diorite, if the hornblende were better 

 characterised, but only doubtful traces of this mineral are to be found in the form of 

 hexagonal sections which might have been amphibolic originally, but are now only 

 pseudomorphs. These sections are almost as large as those of felspar, the contours 

 being sometimes clearly defined ; in other cases they merge into the surrounding ground- 

 mass. With polarised light the mineral in question behaves like an aggregate ; some 

 indistinct patches take a bluish tint or remain unaffected, and these might possibly be 

 nepheline or apatite. Secondary cpiartz is developed, but not to such an extent as 

 epidote, which appears to penetrate the whole mass ; its grains, although often very 

 small, are recognisable by a slight citron-yellow pleochroism, brilliant colours of 

 polarisation, and an irregular surface. It is abundant in the cavities, where it has 

 crystallised in the form of a fan, and is associated with calcite. Sometimes the nearly 

 circular vesicles, which give the rock the appearance of a Diabas Mandelstein, are filled 

 with these two minerals often associated with chlorite. 



A rolled pebble, reddish brown in colour with dark green grains of augite and 

 white grains of altered felspar, is a clastic rock. This tufa contains all the minerals 

 mentioned in the rocks already described. The ground-mass is made up of small, 

 more or less abundant, crystals of plagioclase, and of lapilli, which are distinctly 

 separated from each other and cemented by a coating of ferric hydrate. The large 

 fragments of felspar, which are seen scattered sporadically through the preparations, 

 have the same optical and crystallographic properties as those described above. These 

 sections are usually rounded, and are partly altered, not however by kaolinisation, 

 but rather by zeolitisation ; instead of small micaceous lamellae with iridescent tints 

 in polarised light, these sections are seen showing a blue colour which extends 

 over pretty large surfaces separated by colourless intervals. Numerous crystals of 

 zeolites are also to be seen in the vesicles of the rock. Epidote has crystallised in the 

 interior of the felspar ; with calcite and chlorite they occasionally entirely fill the 



