Ch. XXIX.] VOLCANIC MOUNTAINS. 623 



EXTERNAL FORM, STRUCTURE, AND ORIGIN OF VOLCANIC MOUNTAINS. 



The origin of volcanic cones with crater-shaped summits has been 

 alluded to in the last chapter (p. 593), and more fully explained in 

 the " Principles of Geology " (chaps, xxiv. to xxvii.), where Yesuvius, 

 Etua, Santorm, and Barren Island are described. The more ancient 

 portions of those mountains or islands, formed long before the times 

 of history, exhibit the same external features and internal structure 

 which belong to most of the extinct volcanoes of still higher antiquity ; 

 and these last have evidently been due to a complicated series of 

 operations, varied in kind according to circumstances ; as, for example, 

 whether the accumulation took place above or below the level of the 

 sea, whether the lava issued from one or several contiguous vents, and, 

 lastly, whether the rocks reduced to fusion in the subterranean regions 

 happen to have contained more or less silica, potash, soda, lime, iron, 

 and other ingredients. 



Y\ r e are best acquainted with the effects of eruptions above water, 

 or those called subaerial or supramarine ; yet the products even of 

 these are arranged in so many ways that their interpretation has given 

 rise to a variety of contradictory opinions, some of which will have to 

 be considered in this chapter. 



Craters and Colder as, Sandwich Islands. — We learn from Mr. 

 Dana's valuable work on the geology of the United States Exploring 

 Expedition, published in 1 849, that two of the principal volcanoes of 

 Sandwich Islands, Mounts Loa and Kea in Owyhee, are huge flattened 

 volcanic cones, about 14,000 feet high (see fig. 693), each equalling 

 two and a half Etnas in their dimensions. 



Fig. 693. 



Mount Loa, in the Sandwich Islands. (Dana.) 



a. Crater at the summit. &. The lateral crater at Kilanea. 



The dotted lines indicate a supposed column of solid rock caused by the lava consolidating 



after eruptions. 



From the summits of these lofty though featureless hills, and from 

 vents not far below their summits, successive streams . of lava, often 

 2 miles or more in width, and sometimes 26 miles long, have flowed. 

 They have been poured out one after the other, some of them in 

 recent times, in every direction from the apex of the cone, down 

 slopes varying on an average from 4 degrees to 8 degrees ; but in some 

 places considerably steeper. Sometimes deep rents are formed on the 

 sides of those conical mountains, which are afterwards filled from 

 above by streams of lava passing over them, the liquid matter in such 

 cases consolidating in the fissures and formino- dikes. 



The lateral crater of Kilauea, b, fig. 693, is between 3000 and 4000 

 feet above the sea-level, or about the height of Vesuvius. It is an 



