Ch. XXIX.] LAVA, AND SCOKUE. 489 



rocks of the same country may have been poured out in the open 



air 



* 



Although the principal component minerals of subaerial lavas are the 

 same as those of intrusive trap, and both the columnar and globular 

 structure are common to both, there are, nevertheless, some volcanic 

 rocks which never occur in currents of lava, such as greenstone, the 

 more crystalline porphyries, and those traps in which quartz and mica 

 appear as constituent parts. In short, the intrusive trap rocks, forming 

 the intermediate step between lava and the plutonic rocks, depart in 

 their characters from lava in proportion as they approximate to granite. 



These views respecting the relations of the volcanic and trap rocks will 

 be better understood when the reader has studied, in the 33d chapter, 

 what is said of the plutonic formations. 



EXTERNAL FORM, STRUCTURE, AND ORIGIN OF VOLCANIC MOUNTAINS. 



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

 ded to in the last chapter (p. 462), and more fully explained in the 

 " Principles of Geology" (chaps, xxiv. to xxvii.), where Vesuvius, Etna, 

 Santorin, and Barren Island are described. The more ancient portions of 

 those mountains or islands, formed long before the times of history, ex- 

 hibit 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 re- 

 duced to fusion in the subterranean regions happen to have contained 

 more or less silica, potash, soda, lime, iron, and other ingredients. 



We 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 con- 

 sidered in this chapter. 



Craters and Calderas, Sandwich Islands. — We learn from Mr. Dana's 

 valuable work on the geology of the United States' Exploring Expedition, 

 published in 1849, that two of the principal volcanoes of the Sandwich 

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

 cones, about 14,000 feet high (see fig. 640), each equalling two and a half 

 Etnas in their dimensions. 



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 to the cone, down slopes varying 



* Syst. of GeoL voL ii. p. 1 14. 



