^o Tahiti. 



TAUT I. 



been succeeded by the steam ; but I was not able to 

 ascertain how long this was ago, or anything certain on 

 the subject. When viewing the spot, I imagined that 

 the injection of a large mass of rock, like the cone of 

 phonolite at Fernando Noronha, in a semi-fluid state, by 

 archiug the surface might have caused a wedge-shaped 

 hollow with cracks at the bottom, and that the rain- 

 water percolating to the neighbourhood of the heated 

 mass, would during many succeeding years be driven 

 back in the form of steam. 



Tahiti (OtoJieite). — I visited only a part of the 

 north-western side of this island, and this part is en- 

 tirely composed of volcanic rocks. Near the coast there 

 are several varieties of basalt, some abounding with 

 large crystals of augite and tarnished olivine, others 

 compact and earthy, — some slightly vesicular, and 

 others occasionally amygdaloidal. These rocks are 

 generally much decomposed, and to my surprise, I found 

 in several sections that it was impossible to distinguish, 

 even approximately, the line of separation between the 

 decayed lava and the alternating beds of tuff. Since 

 the specimens have become dry, it is rather more easy 

 to distinguish the decomposed igneous rocks from the 

 sedimentary tuffs. This gradation in character be- 

 tween rocks having such widely different origins, may I 

 think be explained by the yielding under pressure of 

 the softened sides of the vesicular cavities, which in 

 many volcanic rocks occupy a large proportion of their 

 bulkc As the vesicles generally increase in size and 

 number in the upper parts of a stream of lava, so would 

 the effects of their compression increase; the yielding, 

 moreover, of each lower vesicle must tend to disturb all 

 the softened matter above it. Hence we might expect 

 to trace a perfect gradation from an unaltered crystal- 

 line rock to one in which all the particles (although 



