chap. ii. Tahiti. 3 1 



originally forming part of the same solid mass) had 

 undergone mechanical displacement ; and such particles 

 could hardly be distinguished from others of similar 

 composition, which had been deposited as sediment. 

 As lavas are sometimes laminated in their upper parts, 

 even horizontal lines, appearing like those of aqueous 

 deposition, could not in all cases be relied on as a 

 criterion of sedimentary origin. From these considera- 

 tions it is not surprising that formerly many geologists 

 believed in real transitions from aqueous deposits, 

 through wacke, into igneous traps. 



In the valley of Tia-auru, the commonest rocks are 

 basalts with much olivine, and in some cases almost 

 composed of large crystals of augite. I picked up some 

 specimens, with much glassy feldspar, approaching in 

 character to trachyte. There were also many large 

 blocks of vesicular basalt, with the cavities beautifully 

 lined with chabasie (?), and radiating bundles of meso- 

 type. Some of these specimens presented a curious 

 appearance, owing to a number of the vesicles being 

 half filled up with a white, soft, earthy mesotypic 

 mineral, which intumesced under the blowpipe in 

 a remarkable manner. As the upper surfaces in all 

 the half-filled cells are exactly parallel, it is evident 

 that this substance has sunk to the bottom of each cell 

 from its weight. Sometimes, however, it entirely fills 

 the cells. Other cells are either quite filled, or lined, 

 with small crystals, apparently of chabasie ; these 

 crystals, also, frequently line the upper half of the cells 

 partly filled with the earthy mineral, as well as the 

 upper surface of this substance itself, in which case the 

 two minerals appear to blend into each other. I have 

 never seen any other amygdaloid l with the cells half 



1 MacCulloch, however, has described and given a plate of 

 1 Geolog. Trans.,' 1st Series, vol. iv. p. 225) a trap rock, with cavities 



