126 Galapagos Archipelago. paei t 



the surrounding matrix the small gTains of this mineral 

 were tolerably well crystallised. Although there is so 

 wide a difference in appearance between the lava of 

 the little streams, and especially of their red scoriaceous 

 crusts, and one of these angular ejected fragments. 

 which at first sight might readily be mistaken for 

 syenite, yet I believe that the lava has originated from 

 the melting and movement of a mass of rock of abso- 

 lutely similar composition with the fragments. Besides 

 the specimen above alluded to. in which we see a 

 fragment becoming sliffhtly cellular, and blending* into 

 the surrounding matrix, some of the grains of the steel- 

 blue augite also have their surfaces becoming very 

 finely vesicular, and passing into the nature of the 

 surrounding paste ; other grains are throughout, in an 

 intermediate condition. The paste seems to consist 

 of the augite more perfectly fused, or. more probably, 

 merely disturbed in its softened state by the movement 

 of the mass, and mingled with the oxide of iron and 

 with finely comminuted, glassy albite. Hence pro- 

 bably it is that the fused albite, which is abundant 

 in the pinnacle, disappears in the streams. The albite 

 is in exactly the same state, with the exception of 

 most of the crystals being smaller in the lava and in 

 the embedded frao-ruents ; but in the fragments thev 

 appear to be less abundant : this, however, would 

 naturally happen from the intumescence of the augitic 

 base, and its consequent apparent increase in bulk. It 

 is interesting thus to trace the steps by which a com- 

 pact granular rock becomes converted into a vesicular, 

 pseudo-porphyritic lava, and finally into red scoriae. 

 The structure and composition of the embedded frag- 

 ments show that they are parts either of a mass of 

 primary rock which has undergone considerable change 

 from volcanic action 5 or more probably of the crust of 



