RISE OF MOLTEN ROCK TO THE EARTH'S SURFACE 145 



This close resemblance is probably of deep significance, for the 

 reason that shales and slates are structurally the weakest of all 

 rocks and for the further reason that they rather generally di- 

 rectly underlie the carbonate rocks, which are by contrast the 

 strongest (see ante, p. 37). For these reasons shales and slates are 

 the only rocks which are likely to be fused by relief from load 

 through the formation of anticlinal arches within the earth's zone 

 of flow. If this view is well founded, lavas and other igneous 

 rocks are in large part fused argillaceous sediments formed in con- 

 nection with the process of folding, or are refused rocks of igneous 

 origin and similar composition. 



Character profiles. The character profiles 4 of features con- 

 nected in their origin with volcanoes are particularly easy to 

 recognize, and in a few cases in which they might be confused with 

 others of a different origin, an examination of the materials of 

 the features should lead to a definitive judgment. 



The lava plains which result from massive outflows of basalt 

 might perhaps strictly be regarded as lack of feature, so great may 

 be their continuous extent. Wherever definite vents exist, a 

 broad flat dome is the usual result of the extravasation of a basal- 

 tic lava. The puys of France and many of the Kuppen of Ger- 

 many, being formed from less fluid lava, have afforded profiles 

 with relatively small radius of curvature. 



In its youthful stage, the cinder cone usually presents a broad 

 summit sag and relatively short side slopes, whereas the cone of 

 later stages is apt to present long sweeping and upwardly concave 

 curves with both the gradient and the radius of curvature increas- 

 ing rapidly toward the summit. In contrast, too, with the earlier 

 stage, the crest is relatively small. A marked reduction in the 

 high symmetry of such profiles is noted wherever a breaching by 

 lava outflow has occurred (Fig. 154). 



With the composite cone, complexity and corresponding lack 

 of symmetry is introduced, especially in the partially ruined 

 caldera, and by the more or less accidental distribution of parasitic 

 cones, as well as by migrations of the central cone. Peculiarly 

 similar acuminated profiles result from spatter-cone formation, 

 from the formation of a superchimney spine, and by the uncover- 

 ing of the chimney through denudational processes the volcanic 

 neck. 



