COMPARISON WITH OTHER YOLCAXOES. 235 



conditions at present existing in the active basaltic volcano of Etna and 

 those which probably obtained in the basaltic volcano of Crandall Basin. 

 In the last-named region, as it is now exposed, four-fifths of the volcanic 

 material is fragmentary ejectamenta, forming subaerial breccias of angular 

 pieces of massive and scoriaceous lava with tuff or dust. But in the upper 

 parts of the outlying mountains massive flows of lava predominate. In the 

 case of Mount Etna it is known that the central mass, as exposed in the 

 Val del Bove, is mostly fragmentary ejectamenta, but the surface of the vol- 

 cano consists of lava flows to a very great extent. 



In Vesuvius there is a cone of much steeper slope, consisting largely 

 of tuff-breccias, which dip at high angles in the slopes of Monte Somma. 

 The latest eruptions, which form the cone of Vesuvius, have been quiet 

 outflows of lava. A meridional profile of Vesuvius on the same scale as 

 that of Etna is placed under the latter for comparison (PI. XXXII). 



It is to be remarked that the siibaerial breccias and tuffs of Monte 

 Sonnna, while differing in mineral and chemical composition from those of 

 the Crandall district, resemble them most closely in outward appearance 

 and in the manner of their agglomeration. The subaerial breccias of the 

 Yellowstone Park and its vicinity are for the most part more compact than 

 those of Monte Somma, but exhibit the same structure. Many of them, 

 however, present the same degree of cohesion and all the characteristics of 

 recent ejectamenta, 



The volcanoes of the Hawaiian Islands are said to be the results of 

 non-explosive eruptive action, very little fragmentary material entering- into 

 the construction of the mountains. But it must be borne in mind that the 

 central portions of the great volcanoes there are not exposed and their true 

 character is not definitely known. 



Assuming that the volcano which must have existed in the region of 

 Crandall Basin resembled closely the type represented by Etna, and neglect- 

 ing the erosion which undoubtedly removed material from above the outlying 

 peaks of horizontally bedded lavas, we may gain some idea of the original 

 form and proportions of this volcano by constructing above the profile 

 sections through its center the outline of Etna, as represented in the plate 

 of sections. Placing the crater of Etna over the center of the dikes of 

 Crandall Basin (PI. XXXII), and allowing its outer slopes to rest on the sum- 

 mits of the surrounding mountains, we obtain theoretical elevations of the 



