HEAT VOLCANOES. 



689 



— a great depressed area in the surface of the mountain, like a pit 

 or quarry-hole in a plain, as in the summit-crater of Mt. Loa and 

 in Kilauea, the latter 4000 feet above the sea. A larger bird's-eye 

 view of Kilauea (with an adjoining small crater, r) is shown in fig. 969, 

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

 970. The pits have precipitous walls of stratified rocks ; for the 

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



Fig. 970. 



Vertical section of crater of Kilauea, 1840. 



At Mt. 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, 71 miles in circuit, nearly 

 four square miles in area, and 600 to 1000 feet deep, — the latter after one of its great 

 eruptions. It is as much open to the day as a city of two miles square would be 

 within an encircling wall of 600 feet (the present depth) ; and the pools of boil- 

 ing lavas and vapors (one of which is at a, fig. 969) may be as leisurely sur- 

 veyed from the brink as if the objects were gardens and cathedrals. 



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

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

 tufa-cones, therefore, have commonly an angle of between 15 and 30 

 degrees. The layers usually slope inwards towards the bottom of 

 the crater (fig. 971), as well as outwards down the sides. The tufa 



Fig. 971. 



Fig. 972. 



Section of a tufa-cone. 



Assumption Island, one of the Ladrones. 



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 minerals 

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

 (p. 65). The crater has generally a saucer shape. A tufa-cone on 

 Oahu (called Diamond Hill) has a height of 1000 feet. Such cones 

 are among the results of lateral eruptions about a great volcano 

 near the sea. 



3. Cinder-cones. — Falling cinders, like sand, may make a declivity 

 of 40 to 45 degrees. The eruption of cinders, therefore, produces a 



45 



