94 MICHIGAN ACADEMY OF SCIENCE. 



must restore them in imagination to reco.anize tlieir original character, 

 and in ishiuds luithci- nortli the ruins of the original volcanic structure 

 are so crumbled that their rehabilitation is a task that can be accom- 

 plished only by prolonged patient study. 



The beginnings of a volcanic island must be generally beyond the 

 reach of scientific observation. Deep beneath the ocean lies the actual 

 vent marking the crest of a fold in the earth's crust. Under stress of 

 a titanic force, into whose origin we do not now seek to inquire, the 

 crust is crum]iled and is liable to become fissured where it is folded. 

 A mile perhaps below the surface of the ocean, lava is forced out of 

 such fissure. At this depth no explosion will result from contact of 

 the molten rock with the water. The lava will be chilled very quickly, 

 of course, but it will no d(»ubt alisorb a certain amount of water, if it 

 has not already done so in its passage through the fissure, and so will 

 remain fluid at a much lower temperature than it would otherwise. 

 However, at best, the lava must congeal before it can flow to any distance 

 from the vent. 



Again and again lava will be emitted at or close to the original vent, 

 building up thus a submarine mountain, much more steep than any 

 subaerial lava cone. In time this is built up to a point where the 

 diminished jiressure permits the formation of steam, and now begins 

 a new phase in the volcanic phenomena. The action assumes an explo- 

 sive character. Repeatedly the top of the cone is blown to frag:ments 

 and it is a long time before the lava can come to the light of day. By 

 this time there will have been formed a platform of some size on which 

 to build up the new island — a foundation, however, which must consist 

 largely of fragmental matter, piled up at the steepest angle at which 

 it will stand. 



The time comes when a sufficient quantity of lava is forced out at once 

 to build up a subaerial crater with a conduit through which the lava 

 may reach the surface without meeting with water enough to produce 

 a disastrous explosion. Often enough even after this explosions do 

 occur and the island may be repeatedly destroyed thereby — the founda- 

 tion platform made thus broader and more secure. 



I need not describe in detail the process by which the island when 

 once established groAvs, through successive eruptions. I wish, however, 

 to impress the idea of instability in the original foundation, and to call 

 attention to the consequences which must follow from the pouring into 

 the sea of the molten lava. It is evident that this lava will be rapidly 

 chilled and will pile up very close to the shore instead of continuing 

 under water the low-pitch gradient of the cone as it appears above the 

 water. The submarine poition of the island, so far as it is made from 

 lava, must be of a most insecure structure, its material largely frag- 

 mental and piled at so steep an angle as to involve great instability. 

 In many cases there will be also quantities of solid matter ejected in 

 explosive eruptions, all fragmental and much of it in a fine state of 

 division. This may be thrown to a considerable distance, and so may 

 enlarge materially the foundation particularly on the leeward side 

 of the island. The base may, indeed, sonunimes be buttressed and 

 strengthened by submai-ine extrusions of lava, which will certainly 

 sometimes find the path of least resistance leading through this loose 

 material. 



