234 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



One of the largest and most' thoroughly investigated active volcanoes 

 is Etna, whose majestic cone rises from the sea to a height of 10,835 feet. 

 According to the elaborate study of Etna by Sartorius von Waltershausen, 

 as completed by von Lasaulx, 1 the central mass and basal portion of this 

 mountain consist of thick beds of tuff (or breccia) traversed by dikes of 

 massive rock, filling clefts which are for the most part nearly vertical. 

 Between the layers of tuff or breccia are sheets of crystalline rock. These 

 are partly connected with the dikes, and have been injected horizontally in 

 the tuff, or they are in part surface flows of lava. The beds in the central 

 part of the mountain dip as steeply as 29°, and show by their positions and 

 by the different groups of radiating dikes that the centers of eruption 

 have been shifted from southeast to northwest. The lower flanks of the 

 volcano are composed of more numerous lava flows, and slope at angles 

 of from 2° to 5°. These lavas have been mostly erupted from lateral or 

 parasitic cones, the arrangement of which on the surface proves that they 

 all belong to groups in more or less straight lines which exhibit an exactly 

 radial direction from the center of eruption — that is, the present crater of 

 the volcano. In the earlier period of the building of the mountain, as at 

 present, fissures were formed from the center outward, radially. These 

 fissures often lay close together, and were then almost parallel; through 

 them the molten lavas rose and formed dikes; and where they reached the 

 surface they gave rise to parasitic cones. Modern fissures that are radial 

 to the present crater are those of 1669, 1792, 1811, 1852, 1865, 1874, and 

 1879. There are many others which are approximately radial, and others 

 exhibiting no such arrangement. The lava flows on the flanks of the 

 volcano have reached the surface through the radial fissures connected with 

 the central conduit, 



The profile of Mount Etna along a vertical section through the summit, 

 from Catania to Randazzo, drawn to natural scale, is shown in PI. XXXII. 

 The scale is the same as in the profiles across Crandall Basin — rdm. 

 The diameter of the volcano in the direction taken is about 2 7 miles. 



Etna is of very recent age, geologically considered, for its lowest rocks 

 rest on Diluvial, or Pleistocene, deposits, since which period it has piled 

 up scoria and lava to a height of nearly 11,000 feet. 



From the foregoing there appears to be a close analog}" between the 



1 Der ^Etna, Leipzig, 1880. 



