284 DYNAMICAL GEOLOGY. 



same metliods, the floor in a volcano like Vesuvius may be raised, preparatory 

 for another eruption. 



This principle in volcanic science, first made out by C. S. Lyman, is established by 

 facts observed by him on Hawaii. In May, 1840, an eruption emptied the crater of Kilauea, 

 and left it with two thirds of the floor sunken nearly 400 feet below the level which it had 

 just before the eruption. (This was 6 months before the author's first visit to the crater.) 

 Fig. 245 is a transverse section of the crater as it was after the eruption, mo^ o' m being the 

 opposite walls, and np, p'n' the sunken central region or " lower pit.'''' Six months later, 



245. 



Vertical section of crater of Kilauea in 1840. D. '49. 



the walls of the lower pit, which were then 360 feet high, had a talus of broken lava along- 

 side, falls of the rocks being frequent at the time. In 1846, C. S. Lyman found the lower 

 pit of the crater obliterated, and the talus, at the foot of its walls, constituting a ridge 

 100 to 150 feet high. Its floor had been raised as a whole, with the talus of lava-blocks 

 upon it ; and fault-planes made in the sinking of the floor at the eruption in 1840 were 

 those used in the rise. This ridge was gradually buried by the outflow of lavas over the 

 floor, but it still existed in 1864, as shown in the view of Kilauea on page 270, and also 

 in a map of the crater of that date by W. T. Brigham. 



At times of approaching eruption, the heat and projectile action of the 

 crater become intense. , The heat may be expended, as in Kilauea, in multi- 

 plying lava-lakes for ebullition and raising blowing-cones, or, on the other 

 hand, as in Vesuvius, in projecting cinders to enormous heights besides start- 

 ing some lava-flows. 



(6) The eruption. — The eruption begins after the lavas have risen within 

 the crater up to what may be called high-lava mark ; and when the pressure 

 from the vapors generated and confined below and from the hydrostatic pres- 

 sure of the lava-column — chiefly the former — is too great to be withstood 

 by the containing mountain. The mountain consequently breaks ; the con- 

 duit is rent open on one side or the other, and the lavas run out. If the 

 mountain were too strong to break, as it perhaps is in the earlier part of its 

 history, when it is of little height, the lava would rise to the top of the crater 

 by the methods described, and overflow from the summit on this side or that. 

 But modern eruptions, as has been stated, are usually through fissures. ' 



The discharge of the lavas empties the upper part of the lava-conduit or 

 lowers the level of its upper surface, and undermines the lifted crater-floor ; 

 and the result may be (1) a collapse or down-plunge of the floor within the 

 crater, making again a pit hundreds of feet deep, or 1000, or 2000, as the case 

 may be ; and (2) sometimes also a down-plunge of the walls of the crater. 



Part of the undermining at Vesuvius is due to outflow of lavas, part to 

 discharge of volcanic cinders ; but at basaltic Kilauea, it all comes, ordinarily, 

 from the escape of liquid lavas. 



