46 PHYSICAL GEOLOGY 



ORIGINAL STRUCTURES OF ROCKS 



By rock structure is meant the mode of occurrence of rocks, 

 the shapes of rock bodies, and the position or attitude of those 

 bodies. Thus, to say that certain rocks are stratified and 

 that the beds are horizontal, or tilted, or folded, is to state 

 phases of their structure. The principal original structures 

 of rocks are discussed below, while some structures developed 

 by the changes which take place in the outer part of the earth 

 are described in the next Chapter. 



Volcanic cones. The greater part of the material extruded 

 by volcanoes accumulates near the vents, forming conical ele- 

 vations. These cones vary in size and shape. They range 



in height from a comparatively 

 few feet up to high mountains 

 like Mauna Loa in Hawaii, 

 whose summit is some 14,000 

 feet above the neighboring sea 

 and about 30,000 feet above 

 the sea floor. The slope of a 

 cone is determined by its com- 



n,^-u ir } ? wens position. Coarse, angular cm- 

 Valley, Cal. (Fairbanks.) . J? 



ders stand in steep piles (Figs. 



20 and 21), while the more liquid lavas spread freely and build 

 cones whose sides in exceptional cases form angles of only 

 two or three degrees with a horizontal plane (Fig. 22). Stiffer 

 lavas form cones of intermediate steepness. Lava cones 

 consist of many solidified streams of lava which flowed from 

 the vent in different directions at different times, producing a 

 sort of radial structure. Most cones, like Vesuvius, are built 

 of both lava and fragmental material, and for various reasons 

 they are often irregular in form and structure. 



A majority of the existing volcanic mountains are near the 



