144 



EARTH FEATURES AND THEIR MEANING 



bined fracture and flow, would be sufficient to remove the load 

 from relatively weak underlying beds, which in consequence would 

 be fused and form local reservoirs of lava (Figs. 152 and 153). 



It has been further quite generally observed that lines of vol- 

 canoes, in so far as they betray any relation in position to neigh- 

 boring mountain ranges, tend to appear upon the rear or flatter 

 limb of unsymmetrical arches, or where local tension would favor 

 the opening of channels toward the surface. Moreover, wherever 

 recent block movements of surface portions of the earth's shell 

 have been disclosed in the neighborhood of volcanoes, the latter 

 appear to be connected with downthrown blocks, as though the lava 

 had, so to speak, been squeezed out from 

 beneath the depressed block or blocks. 



We must not, however, forget that the 

 igneous rocks are greatly restricted in the 

 range of their chemical composition. No 

 igneous rock type is known which could 

 be formed by the fusion of any of the 

 carbonate rocks such as limestone or 



patent 

 Willis). 



formation (after 



FIG. 153. Result of experi- 

 ment with layers of com- 

 position to illustrate the 



effect of relief of load upon dolomite, or of the more siliceous rocks, 

 rocks by arching of com- such as sandstone or quartzite. There 

 remains only the argillaceous class of 

 sediments, the shales and slates, and so 

 soon as we examine the composition of these rocks we are struck by 

 the remarkable resemblance to that of the class of igneous rocks. 

 For purposes of comparison there is given below the composite or 

 average constitution of igneous rocks in parallel column, with the 

 average attained by combining the analyses of 56 slates and shales, 

 the latter recalculated with water excluded: 



