232 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 19, 



We may naturally ask whether M. de Beaumont's notiou of the 

 existence of such linear Tents as the dikes above alluded to, is borne 

 out by the analogy of the pheenomena of other active volcanos. Mr. 

 DarAA-in tells me that in St. Jago he saw horizontal sections of the 

 bases of small craters, and the mass of rock which had formed the 

 source or feeder was of a circular, not a linear form. He has also 

 given us the section of a cone of eruption in the Galapagos (Volcanic 

 Islands, p. 109), where we have a most perfect natural dissection of 

 a crater. In that case we see a series of inclined parallel beds of ba- 

 salt, separated by beds of loose, fragmentary scoriee, all parallel, and 

 very uniform. Three of the sheets of lava unite with an irregular mass 

 or column of the same substance, which was evidently the axis of the 

 crater. The other streams of lava were no doubt, says INlr. Darwin, 

 origmally united to the same column, before it was worn down by the 

 sea. Such a junction bears no resemblance to the dikes in the Yal 

 del Bove, because the lava has risen up a circular crater, and not by 

 a linear fissiu'e, and a cone has been formed ; whereas it is precisely 

 the absence of such small cones connected with dikes on Etna, which 

 presents the difficulty to which I now allude. 



Since the above remarks were written, I have perused Mr. Dana's 

 valuable work, on the Geology of the United States' Exploring Expe- 

 dition, published in 1849, and which reached London after this paper 

 was drawn up. His observations on the great volcanos of the Sand- 

 wich Islands tend greatly to confirm my views, in regard to the forma- 

 tion of large flattened domes of volcanic matter poured out from a 

 central vent, and they show that wide and extensive sheets of compact 

 basalt and greystone have been formed on slopes considerably exceed- 

 ing those which M. de Beaumont thought possible. In two of the 

 principal volcanos of Owyhee, for example. Mounts Loa and Kea, we 



Fig. 11. 

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



a. Crater at the summit. 6. Crater of Kilanea. 



have examples of huge flattened volcanic cones 15,000 feet high (see 

 fig. 11), each equahng two and a half Etnas in their dimensions, from 

 the summits of which, and from vents not far below the summit, suc- 

 cessive streams of lava, two miles or more in width, and sometimes 

 twenty-six miles long, have been evolved. They have been poured 

 one after the other in every direction from the apex of the cone, dovm 

 slopes varying on an average from 4° to 8°, but in some places consi- 

 derably exceeding that inclination. INIr. Dana, indeed, convinced him- 

 self from actual observation, that, owing to the suddeimess with which 

 the lava cools, it may occasionally form on slopes equaUng 25°, and 

 still preserve considerable solidity ; nay, it is even, he says, possible, 

 from what he saw in the great lateral crater of Kilanea (fig. 11^), 



