862 PRINCIPLES OF STRATIGRAPHY 



obliterated and the adjoining plateau elevated, while the upper and 

 deeper portion near the center has been almost filled by ejecta. 

 The dust-flows are the material left behind by the dust-laden clouds 

 of steam. The exploding clouds of steam were so overloaded with 

 dust and larger fragments of comminuted lava, that they flowed 

 down the slope of the mountain and the gorge, like a fluid propelled 

 at a high velocity by the horizontal or partly downward component 

 of the force of the explosion. Many large fragments of solidified 

 lava were carried down the gorge by these clouds. Such blocks 

 lo to 15 feet in diameter were not uncommon" (iy:^6o). 



Lapilli vary in size from that of a walnut to. dust. The term 

 is somewhat loosely used, and should be restricted to pyrogenic 

 material in a state of division, i. c, pyrogranulytes and the smaller 

 pyrosphasrytes (more rarely pyro-pulverytes) which by their ap- 

 pearance show that they were unconsolidated or at least in a plastic 

 state on eruption. (See ante, Chapters \T and XII.) The sand 

 and dust are, for the most part, true pyroclastic material character- 

 ized by angularity of outline and density of material. 



The forms of cinder cones. Cinder cones are essentially local 

 accumulations of unconsolidated materials, and so their form is 

 determined by the general laws which govern the accumulation of 

 such material, modified, of course, by the special influences char- 

 acteristic of the mode of accumulation. The form will also vary 

 in accordance with the prevailing size and character of the material, 

 being steeper for coarse and gentler for fine material, and gentler 

 also for rounded material (lapilli) than for angular. There will 

 be further variation induced by the abundance or scarcity of water 

 vapor, condensed into rain in the vicinity of the eruption, the 

 variation being analogous to that found in the slopes of alluvial 

 cones and dry "cones of dejection," or between that of alluvial 

 cones of dry and pluvial regions. '"Speaking broadly, the diameter 

 of the crater is a measure of the violence of the explosion within 

 the chimney. A single series of short explosive eruptions builds a 

 low and broad cinder cone. A long-continued succession of moder- 

 ately violent explosions, on the other hand, builds a high cone with 

 crater diameter small if compared with the mountain's altitude, and 

 the profile afforded is a remarkably beautiful sweeping curve." 

 (Hobbs-15 :i2j.) Owing to the fact that material near the summit 

 lies at the maximum angle of repose, while that lower down gener- 

 ally has a lower angle — the product of change wrought by time 

 and by the addition of material fallen from the sky upon the 

 surface of the original slope — the form of the lateral curve of the 

 cinder-cone will be a faintly concave one, whereas that of a lava 



