452 W. T. BRIGHAM ON THE VOLCANIC PHENOMENA 



depths, is kept so uniform we do not know ; but we do know that the relative level of the 

 land and sea is constantly changing, generally, as on the coast of Norway and the eastern 

 coast of the United States, raising the land gradually at the rate of a few feet in a century, 

 or in a similar way depressing the bed of the Pacific, the Baltic, and a part of the Indian 

 Ocean. Or the change may be sudden, as during an earthquake. 1 



The bed of the ocean seems to be sinking and the land rising ; but our knowledge of 

 the earth's surface is yet too limited to inform us how extensively this is the case. The 

 Hawaiian Islands are rising, and the atolls in the same ocean are sinking ; and if we admit 

 that the shallow bed of the Pacific, extending from the Hawaiian Islands to the China Sea, 

 was once a continent, we have a case the reverse of those previously cited. 



Wherever this change of level occurs over a tract of greater or less extent, there will be 

 cracks of greater or less size along the boundary of this area ; and here will the contracting 

 crust force the lavas beneath, through to the surface. This fact was long since recognized 

 by Humboldt, who says: " I am inclined to believe that islands and coasts are only richer in 

 volcanoes, because the upheaval effected by internal elastic forces is accompanied by depres- 

 sion of the bed of the adjacent sea ; so that an area of elevation borders on an area of 

 subsidence, and at the limit between these areas great and profound clefts and fissures are 

 occasioned." 2 



If the continents were not rising it would be difficult to see how they could maintain any 

 height above the general surface of the globe, for meteoric influences constantly degrade 

 and lower their peaks and slopes, and the material taken from them is constantly poured 

 into the bed of the ocean, tending to fill that up, and reduce all to a common level. 



The theory of a contracting crust, although not based on a sufficient knowledge of cosmic 

 forces, yet seems to satisfy the known condition of volcanoes better than any other ; and it 

 also explains earthquakes which have been repeatedly shown to be unnecessary to eruptive 

 action. 



This theory, briefly expressed, is this. The earth's crust contracts unequally owing to its 

 various composition, structure, and form, causing certain portions to fall below the general 

 level, opening rents at the boundaries, and forcing up molten matter to the surface. The 

 vibration of this gradual change of level, and consequent disruption of beds of rock, giving 

 rise to earthquakes. 



Mineral Products of the Hawaiian Volcanoes. 



In the classification of the Hawaiian minerals I have followed the arrangement of Prof- 

 Dana. The rocks, however, present some difficulties, since the metamorphic influences are 

 both continual and considerable. The usual division of lavas into basaltic and trachjtic is most 

 unsatisfactory, since the lavas under consideration are mostly intermediate, of the clinkstone 

 variety. If we attempt to draw a line between the recent and ancient, there are lavas yet 

 hot which can hardly be distinguished from some that have been exposed to meteoric in- 

 fluences for centuries. Nor is the chemical composition sufficiently constant to serve as a 

 basis of classification. 



1 The island of Tongatabu in the Friendly Group, during a the sea advanced on the land two miles, while the western 

 recent earthquake, was depressed on the north-east side, so that coast rose several feet. 



2 Kosmos, iv. 415, Sabine's trans. 



