276 



DYNAMICAL GEOLOGY. 



Kilauea are thus elongated and eccentric, and have lines of fissures extending far to the 

 southwest in the direction of the longer axis ; moreover, the former has the larger part 

 of its more recent eruptions either to the north-northeast or south-southwest of the crater. 

 The two craters of Maui are also elongated and eccentric. The position of the present 

 vent of Vesuvius with reference to the original Mount Somma is eccentric, according to 

 Johnston-Lavis, and perhaps for a like reason. 



3. Causes iiifluencing the forms of cinder- and tufa-cones. — Cinder-cones 

 have their forms varied in height, breadth, and slope, on the different sides, 

 by the winds. Moreover, alternations of cinder and lava ejections make 

 a cone of steeper slope than lava alone. Summit ejections of cinders may 

 increase the height without adding much to the mass of a mountain. Mount 

 Kea owes its superior height over Mount Loa to a final spurt, when it was 

 becoming extinct, cinder-cones at the top having been then thrown up. 



Flowing volcanic mud, from which tufa and tufa-cones are made, neces- 

 sarily produces broad-topped cones with a saucer-like crater, as explained on 

 page 270 ; but the winds often carry cinders far away to make horizontal 

 deposits, which sometimes attain great thickness. By making an outline of 

 a section of a cone and drawing lines parallel to the sides, as below, sec- 

 tions representing in a general way the structure of a lava-cone, cinder-cone. 



239. 



240. 



Cinder-cone. 



Tufa-cone. 



and tufa-cone are easily made. But it is to be noted that such sections are 

 incorrect, since lava streams and cinder deposits are, to a large extent, strips 

 or patches over the surface of the cone and not a series of seamless coats. 



4. Relations of glassy lavas to the stony. — Glassy forms of lava (see page 

 77) occur with each of the three kinds, but make no cones. With basaltic 

 lavas they constitute merely a crust on a lava stream, or the scum of a lava 

 lake ; but in a trachytic volcano, the glass, called obsidian, sometimes flows 

 in streams. 



"Obsidian Cliff" in the Yellowstone Park is a remarkable example of an obsidian 

 outflow. It has the columnar forms of Fig. 224. The glass is connected with vast erup- 

 tions of rhyolyte (quartz-trachyte) at and about Mount Washburn, which have a thickness 

 of thousands of feet, and succeeded to andesyte eruptions (Iddings, Hague). Another 

 locality has been reported by Russell near Mono Lake, in western Nevada. On the trachyte 

 islands, north of Sicily, Lipari (1601 feet high) and Vulcano (1978 feet), the obsidian 

 streams bear evidence of sluggish twisting flow (Judd, 1875). With the glass occurs 

 pumice, and that of Lipari is the pumice of the arts. The northern island of the group, 

 Stromboli (3090 feet high), is basaltic in its lavas ; the islands intermediate between 

 Stromboli and Lipari have lavas of intermediate kinds. 



