REVIEW OF VOLCANIC ACTION. 355 
summit, three to eight hundred feet in height. ‘These cones are often 
broken down on one or more sides by subsequent eruptions. 
Cinder cones, though abundant over Hawaii and other islands of 
comparatively recent origin, are of rare occurrence upon those of 
less modern eruptions. The denudating agents that could scoop out 
valleys in the solid mountains, easily wash away structures consisting 
of so loose material. 
2. Tufa Cones. 
Tufa cones vary from a few yards to a thousand feet in height; 
and like the cinder cones they occur as subordinates to the great 
craters of the islands. ‘They usually stand near the sea, and often 
form over submarine vents near shores; in all cases they owe their 
peculiarities to ejections of water, or unusual volumes of steam 
along with cinders or earth, as we have illustrated in our remarks 
on Oahu and Hawaii. Though otherwise similar in origin, they 
have, therefore, broader summits than cinder cones, shallower and 
more saucer-like craters, gentler slopes, and a firmer constitution. 
The angle of inclination seldom exceeds thirty degrees, and may be 
quite small; and the structure is often as distinctly and thinly lami- 
nated as river alluvium. The waters falling upon the rising slopes 
prevent that sharpness of edge and steepness of inclination which 
characterize the cinder cone. ‘This cause gives also a yellowish- 
brown colour to the cinders by rendering the iron a hydrate. ‘The 
broad summit, and inward as well as outward dip of the layers, and 
the alluvium-like structure have the same origin. The hills at 
Nanawale, however, show us that the tufa cone may become closed 
at summit, and lose its broad crater; but in this case, it appears that 
the ejections gradually diminished in amount, and assumed nearly a 
dry cinder character. 
These tufa cones have been described as covering lava vents, and 
as consisting sometimes in part of lava,—thus varying in character 
like the cinder cones. 
3. Lava Cones. 
The lofty summits of Loa and Kea, and the hillocks of lava upon 
their sides, are alike examples of lava cones but little altered in gene- 
