

726 



DYNAMICAL GEOLOGY. 



view of Kilauea (with an adjoining small crater, r) is shown in Fig. 



1112, and a vertical transverse section of the same, more enlarged, in 



Fig. 1113. The pits have precipitous walls of stratified rocks; for 



the lavas are in layers, and the layers are nearly horizontnl. 



At Mount Loa, the summit-crater is 13,000 feet in its longer diameter, and 780 feet 

 deep. Kilauea is 16,000 feet in its greatest length, seven and one half miles in circuit, 

 nearly foursquare miles in area, and 600 feet deep. After its last great eruption, of 1840, 

 the pit at centre (Fig. 1113, p p) was 1,000 feet deep, with a ledge around — the " black 

 ledge " — (no n'o') 600 feet down. The crater is as much open to the day as a city of 

 two miles square would be, within an encircling wall of six hundred feet (the present 

 depth) ; and the pools of boiling lavas and vapors (one of which is at a, Fig. 1112) may 

 be as leisurely surveyed from the brink as if the objects were gardens and cathedrals. 



Fig. 1113. 



Vertical section of crater of Kilauea. 1840. 



2. Tufa-cones. — Flowing mud from a boiling basin, or cinders wet 

 with water and steam, will take a larger angle of flow than lavas ; 

 and tufa-cones, therefore, have commonly an angle of between fifteen 

 and thirty degrees. The layers usually slope inward toward the bot- 

 tom of the crater (Fig. 1114), as well as outward down the sides. 



Fig. 1114. Fig. 1115. 



Sectiou of a tufa-cone. 



Assumption Island, one of the Ladvones. 



The tufa has a brownish-yellow color, owing to the action of the steam 

 or hot water on the cinders, oxydizing part of the iron in the miner- 

 als (pyroxene mainly) of the lavas, and making a hydrous sesquioxyd. 

 The crater is generally saucer-shaped. Such cones are among the re- 

 sults of lateral eruptions about great volcanoes near the sea. 



3. Cinder-cones. — Falling cinders may make a declivity of about 

 forty degrees. The eruption of cinders, therefore produces a crater 

 with a narrow throat, a narrow rim above, steep sides, the slope thirty- 

 five to forty degrees (Fig. 1115). If the volcano is in brisk action, 

 the space within the crater is dark with the rising vapors ; and the 

 explosions attending the ejection of cinders occur usually at short in- 

 tervals. 



The cone is at first nearly black or brownish-black ; but, if not soon covered with 

 vegetation, it often becomes, through atmospheric agencies, of a red color, from the 

 oxydation of the iron in the lava: the sesquioxyd of iron formed differs from that of 

 the tufa-cone in not containing water, and hence the difference of color. 



