140 The Distribution of part i. 



ing them, I doubted whether such hair-like and curvili- 

 near veins could have been injected, and I now suspect, 

 that instead of having been injected from the dike, 

 they were its feeders. If the foregoing view of the 

 origin of trap-dikes in widely extended granitic regions 

 far from rocks of any other formation, be admitted as 

 probable, we may further admit, in the case of a great 

 body of plutonic rock, being impelled by repeated 

 movements into the axis of a mountain-chain, that its 

 more liquid constituent parts might drain into deep 

 and unseen abysses ; afterwards, perhaps, to be brought 

 to the surface under the form, either of injected masses 

 of greenstone and augitic porphyry, 1 or of basaltic 

 eruptions. Much of the difficulty which geologists 

 have experienced, when they have compared the com- 

 position of volcanic with plutonic formations, will, I 

 think, be removed, if we may believe, that most plutonic 

 masses have been, to a certain extent, drained of those 

 comparatively weighty and easily liquefied elements, 

 which compose the trappean and basaltic series of 

 rocks. 



On the distribution of volcanic islands. — During 

 my investigations on coral-reefs, I had occasion to 

 consult the works of many voyagers, and I was invari- 

 ably struck with the fact, that with rare exceptions, the 

 innumerable islands scattered throughout the Pacific, 

 Indian, and Atlantic Oceans, were composed either of 



1 Mr. Phillips ('Lardner's Cyclopaedia,' vol. ii. p. 115) quotes Von 

 Buch's statement, that augitic porphyry ranges parallel to, and is 

 found constantly at the base of, great chains of mountains. 

 Humboldt, also, has remarked the frequent occurrence of trap-rock, 

 in a similar position ; of which fact I have observed many examples 

 at the foot of the Chilian Cordillera. The existence of granite in 

 the axes of great mountain chains is always probable, and I am 

 tempted to suppose, that the laterally injected masses of augitic 

 porphyry and of trap, bear nearly the same relation to the granitic 

 axes which basaltic lavas bear to the central trachytic masses, round 

 the flanks of which they have so frequently been erupted. 



