DARWIN ON VOLCANIC ISLANDS. 557 



faults. By this hypothesis, the elevation of the districts in mass 

 and the flowing of deluges of lava from the central platforms are 

 likewise connected together, and the marginal basaltic mountains 

 of the three islands alluded to might thus still be considered as 

 forming ' craters of elevation,' the kind of elevation implied having 

 been slow, and the central hollow or platform having been formed, 

 not by the arching of the surface, but simply by that part having 

 been upraised to a less height." (p. 96.) 



The formation and position of crystals, whether of albite or 

 other volcanic minerals, in basalt, is a subject of considerable in- 

 terest, and is connected with that of the order of eruption when 

 trachyte and basalt are both present. Observations on this sub- 

 ject, and relating to the spontaneous separation of certain minerals 

 and the formation of dykes in partially cooled rocks, are of the 

 greatest interest to the geologist, since almost every region in 

 which igneous rocks are extensively distributed abounds also in 

 trap dykes, and it is no easy matter to determine whence the 

 greenstone and basalt forming these dykes was derived. 



" Are we to suppose, like some of the elder geologists, that a zone of trap is 

 uniformly spread out beneath the granitic series, which composes, as far as we 

 know, the foundations of the earth's crust ? Is it not more probable that these 

 dykes have been formed by fissures penetrating into partially-cooled rocks of 

 the granitic and metamorphic series, and by their more fluid parts, consisting 

 chiefly of hornblende, oozing out and being sucked into such fissures? At Bahia, 

 in Brazil, in a district composed of gneiss and primitive greenstone, I saw many 

 dykes of a dark augieic or hornblendie rock, which, as several appearances 

 clearly proved, either had been formed before the surrounding mass had become 

 solid, or had, together with it, been afterwards thoroughly softened. On both 

 sides of one of these dykes the gneiss was penetrated to the distance of several 

 yards by numerous curvilinear streaks or threads of dark matter, and some few 

 of these threads could be traced to their junction with the dyke. When exa- 

 mining them I doubted whether such hair-like and curvilinear veins could have 

 been injected ; and I now suspect that, instead of having been injected from the 

 dyke, they were its feeders. If the foregoing view of the origin of trap-dykes 

 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 moun- 

 tain' 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, or of basaltic 

 eruptions. Much of the difficulty which geologists have experienced when they 

 have compared the composition of volcanic with plutonic formations, will, I 

 think, be removed, if we believe that most plutonic masses have been, to a cer- 

 tain extent, drained of those comparatively weighty and easily liquified elements 

 which compose the trappean and basaltic series of rocks." (p. 124.) 



The distribution of volcanic islands scattered through the great 

 oceans which cover so large a proportion of the earth, becomes a 

 matter of interest when we consider it with reference to the rest 

 of the islands in those seas and the other volcanic districts of the 

 globe. With the exception of some islands of large size, as New 

 Zealand (which, however, contains volcanoes), the Falkland Islands, 

 near the coast of South America, New Caledonia (a large island), 

 the Seychelles, situated in a line prolonged from Madagascar, and 



vol. i. r p 



