706 



DYNAMICAL GEOLOGY. 



the pit at centre was 1,000 feet deep, with a shelf around of about 600 feet, a condition 

 represented in Fig. 1111. 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. 1110) may be as leis- 

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



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. 1112), as well as outward down the sides. 



Fig. 1112. Fig. 1113. 



St ction of a tufa-cone. 



Assumption Island, one of the Ladrones. 



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

 or hot water on the cinders, peroxydizing part of the iron in the min- 

 erals (pyroxene mainly) of the lavas, and making a hydrous peroxyd. 

 The crater is generally saucer-shaped. A tufa-cone on Oahu (called 

 Diamond Hill) has a height of one thousand feet. Such cones are 

 among the results 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 slojje thirty- 

 five to forty degrees (Fig. 1113). 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 

 peroxydation of the protoxyd of iron in the lava: the perox3 r d of iron formed differs 

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

 The growth of vegetation tends to change back the red color to brownish-black, since 

 the carbon deoxydizes the peroxyd, making protoxyd and carbonic acid. 



4. Mixed Cones. — The cones which, like Vesuvius, are formed 

 partly of lava and partly of cinders or tufa, may have any angle of 

 slope, up to thirty-five degrees. They may be lava below, and termi- 



