Ch. XXIX.] JAVANESE CALDERAS. 62 7 



smrmnt in 1772 ; but affirms that most of the towns on its sides said 

 to have been engulfed were in reality overflowed by lava. 



From the highest parts of many Javanese calderas rivers flow, which 

 in the course of ages have cut out deep valleys in the mountain's side. 

 As a general rule, the outer slopes of each cone are furrowed by 

 straight and narrow ravines from 200 to 600 feet deep, radiating in all 

 directions from the top, and increasing the number as we descend to 

 lower zones. The ridges or "ribs" intervening between these furrows 

 are very conspicuous, and compared to the spokes of an umbrella. In 

 a mountain above 10,000 feet high, no furrows or intervening ribs are 

 met with in the upper 300 or 400 feet. At the height of 10,000 feet 

 there may be no more than 10 in number, whereas 500 feet lower 32 

 of them may be counted. They are all ascribed to the action of 

 running water ; and if they ever cut through the rim of a caldera, it is 

 only because the cone has been truncated so low down as to cause the 

 summit to intersect a middle region, where the torrents once exerted 

 sufficient power to cause a series of such indentations. It appears 

 from such facts, that, if a cone escapes destruction by explosion or 

 engulfment, it may remain uninjured in its upper portion, while 

 there is time for the excavation of deep ravines by lateral torrents. 



It is remarked by Dr. Junghuhn, as also by Mr. Dana, in regard to 

 the Pacific Islands, that volcanic mountains, however large and how- 

 ever much exposed to heavy falls of rain, support no rivers so long as 

 they are in the process of growth, or while the highest crater emits 

 from time to time showers of scoriae and floods of lava. Such ejecta- 

 menta and such currents of melted rock fill up each superficial inequal- 

 ity or depression where water might otherwise collect, and are more- 

 over so porous that no rill of water, however small, can be generated. 

 But where the subterranean fires have been long since spent, or are 

 nearly exhausted, and where the superficial scoriae and lavas decom- 

 pose and become covered with clayey soils, the erosive action of water 

 begins to operate with a prodigious force, proportionate to the steep- 

 ness of the declivities and the incoherent nature of the sand and 

 ashes. Even the more solid lavas are occasionally cavernous, and 

 almost always alternate with scoriae and perishable tuffs, so as to be 

 readily undermined, and most of them are speedily reduced to frag- 

 ments of a transportable size because they are divided by vertical 

 joints or split into columns. 



Canary Islands — Palma. — I have enlarged so fully in the " Prin- 

 ciples of Geology" on the different views entertained by eminent 

 authorities respecting the origin of volcanic cones, and the laws gov- 

 erning the flow of lava, and its consolidation, that, in order not to 

 repeat here what I have elsewhere published, I shall confine myself 

 in the remainder of this chapter to the description of facts observed 

 by me during an exploration of Madeira and some of the Canary 

 Islands in 1853-4. In these excursions, made in the winter of 

 1853-4, I was accompanied by an active fellow-laborer, Mr. Hartung, 



