jxxiii SUBMARINE PALAGONITE TUFFS 329 



^ The fragments of semi-vitreous basic rocks forming a regular 

 constituent of these tuffs are usually dark and opaque and display 

 a few plagioclase lathes. They correspond with the type of the 

 semi-vitreous basalt or basaltic andesite, of which the blocks of the 

 overlying agglomerates are as a rule composed and are doubtless 

 derived from the same source. Fragments of unaltered basic 

 glass are rarely to be observed in these tuffs. It is as a rule all 

 converted into palagonite. This material presents itself in various 

 stages of secondary alteration, from the compact greenish or 

 yellowish waxy mass to a white friable pulverulent substance, 

 which represents the last stage of degradation. These changes 

 will be found described on page 348. It can, however, be stated 

 here that they are mainly concerned with hydration. In the case 

 of the lapilli of finely vesicular basic glass, that is, of basic 

 pumice, which are inclosed in some of the tuffs, all stages of the 

 secondary alteration of palagonite are often exhibited, and the 

 last stage of the change is merely indicated by a white powdery 

 patch containing a few minute siliceous amygdules. The puzzling 

 little white patches so common in basic tuffs merely represent 

 lapilli of basic pumice that have been palagonitised, and then 

 bleached and disintegrated by hydration. 



The minerals are more or less abundant and may constitute a 

 third of the whole mass. They include plagioclase, augite, rhombic 

 pyroxene, and magnetite, olivine being rare and scanty. Entire 

 crystals of any size are infrequent. However, crystals of augite, 

 5 or 6 mm. in length, are found in the tuffs at Naivaka and of the 

 coast cliffs near the Salt Lake Passage. ... It may be observed 

 that zeolitic minerals which are frequently developed in the tuffs 

 consisting almost entirely of palagonite are not typical of the tuffs 

 of mixed composition. 



There is nothing suggestive of recent eruptions in any of these 

 formations. They were formed ages since on the sea-floor at vary- 

 ing depths around volcanic vents. Sometimes a cone was able to 

 rear itself above the level of the sea ; but in most cases it rapidly 

 succumbed to breaker-action. Three agencies, concerned with 

 sub-aerial eruptions, submarine eruptions, and marine denudation, 

 have co-operated in the production of these deposits, but their 

 parts in the process have varied greatly. The last is indicated 

 when the tuff is formed of a variety of basic rocks with but little 

 palagonite. The tuffs containing much palagonite representing an 

 original vacuolar basic glass are regarded as mainly the products 

 of submarine-eruptions. In those cases where lapilli of altered 



