78 BiCHTHOFEN NATURAL SYSTEM 



not be surprised to see, that other rocks composed of silicates, which are so widely sep- 

 arated in time as is the case with granite and rhyolite, should offer even greater differ- 

 ences. Some further clue to a better knowledge of this subject may be expected from 

 the study of the nature of volcanic rocks. We see the same chemical compound 

 forming highly viscid lava in one volcanic crater, while it is quite liquid in another. 

 There, it solidifies to dolerite or leucitophyre ; here, to basalt. Similar influences 

 appear to work greater differences in more silicious compounds. We should therefore 

 put little value on the doubts entertained in regard to the solidification of granite, as of 

 a rock ejected on the surface and expanded over it, until we are better acquainted 

 with the causes of the lithological differences among volcanic rocks ; and the evidence 

 regarding the mode of origin of granite should, till then, be mainly taken from its 

 geological occurrence and its chemical composition. 



RELATION OF THE DISTRIBUTION OF THE VOLCANIC ROCKS TO THE CONFIGURATION OF 



THE SURFACE OF THE GLOBE. 



If we embrace in a broad review all those relations mentioned in the foregoing 

 chapters which regard the history of eruptive action, we arrive at the general conclu- 

 sion that, in reference to the entire globe, it is one homogeneous and harmonious 

 whole, and has been attended by such gradual changes only as were necessarily 

 occasioned by the progress of the physical development of the globe itself, while, in 

 regard to every different part, it presents a series of distinct phases intimately con- 

 nected by mutual relations. It would be a subject worthy of the closest investigation, 

 to trace the effect which the events of these phases have had severally upon the struc- 

 ture and the configuration of any separate country. A comparison of these effects 

 as they are manifested in different regions would then aid in establishing some of the 

 chief causes of the differences of structure peculiar to each ; and the knowledge of 

 these would make us better acquainted with the laws governing the evolution of the 

 globe, and prepare the way for a more thorough understanding of its physical geog- 

 raphy. I will only attempt at this place to trace the effects referred to in regard to 

 the last of the phases of eruptive activity, the only one which was nearly contempo- 

 raneous in all countries. It occurred at a comparatively recent date, and we can view 

 the results in a clearer light than we can those which are remote in time and partly 

 obliterated by the vast changes which since then have revolutionized the surface. 



We have, in the first place, to trace more in detail than we have done before, 

 the peculiarities of the distribution of volcanic rocks. It has been observed that ac- 

 tive volcanoes are chiefly situated along the lines of the present sea-coasts, especially 

 at the foot of mountain ranges parallel to them ; or that they follow elevated sub- 

 marine ranges, when they will either remain submerged beneath the sea, or protrude 

 above it, forming chains of islands. Some of these appear to mark the lines of high 

 mountain ranges bounding submarine continents, if we may use this expression for 

 those areas of a shallow sea-bottom which are separated from others of great depth 

 by wall-like elevations, as are indicated for instance in the main ranges of the vol- 



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