i;o CAUSE OF CRUMPLING OF ROCKS. 



the granites, trachytes, and rhyolites ; the deeper-seated layer, less 

 perfectly combined with oxygen and silica, would correspond with 

 the basic rocks, and was supposed to have yielded the syenites, 

 diorites, basalts, and andesites. 1 Professor Judd 2 has gone a step 

 farther, and assumed that beneath these layers are the unoxidised 

 metals, which are occasionally brought to the surface in volcanic erup- 

 tions. Baron Itichthofen, refining on this principle, recognised five of 

 these successive magmas, which have yielded to the surface as many 

 different kinds of volcanic rocks. All these views, however, imply 

 original igneous fusion, increasing oxidation of the rocks towards the 

 surface, increasing specific gravity on descending beneath the surface, 

 and a thin crust through which the materials could reach the surface. 

 8uch views constitute an important school of geological thought, and 

 they deserve careful consideration. But the whole bearing of field 

 observation leads us to assign another interpretation to volcanic 

 phenomena. 



Hypothesis of Metamorpbic Origin of Igneous Rocks. For a long 

 time only certain granites have been claimed as eruptive, while it has 

 been conceded that in many cases, if not in most, they are meta- 

 morphosed slaty rocks, and have been elaborated in the positions in 

 which they occur. We believe that the so-called eruptive granites 

 will prove to be nothing but metamorphic granites, which have been 

 displaced from the positions in which they were formed, by the forces 

 which brought the rock into existence. If granitic rocks can thus 

 originate independently of the supposed magmas, it is manifest that 

 the liquefied forms of those materials, termed 'volcanic rocks, have also 

 originated as products of metamorphism. Therefore we do not believe 

 that the internal heat of the earth necessarily points to any order in the 

 arrangement of igneous materials beneath the surface, or furnishes a 

 logical explanation of the phenomena of volcanic energy. 



Theoretical Influence of Radiated Heat on the Earth's Surface. 

 The heat, however, which is radiated seems more competent to give 

 rise to physical changes in the earth's envelopes, which connect the facts 

 of metamorphism, of rock structure, and igneous phenomena, so as to 

 explain their existence in a systematic way. For since nearly every 

 known substance contracts in dimensions on cooling, it follows that 

 in descending beneath the surface the rocks must be now undergoing 

 a process of contraction. At the surface, where cooling is complete, 

 contraction is complete. But if a warmer layer existed beneath this 

 surface layer and inseparable from it, and cooled subsequently, it must 

 contract, so as to draw the upper layer together laterally and throw it 

 into folds. If we further suppose the cooling to extend deeper and 

 deeper, then each successive addition of a new contracting layer 

 below will increase the number and intensity of the folds and contor- 

 tions of the surface rocks. And since the surface rocks are thus 

 caused to occupy less horizontal space, they necessarily become 

 variously crumpled, elevated, and depressed. This conception of 

 plication of the earth's crust was first enunciated by Constant Prevost. 

 1 See Haughton's " Geology :" Appendix. 2 "Volcanoes." 



