118 R. M. Deeley — Mountain Building. 



that the eruption or welling up of these enormous volumes of lava 

 vras not of a violent character. There are many cases also where 

 huge blocks of country,' bounded by faults, have subsided, and the 

 liquid magma below has issued along the fault lines. 



We must, therefore, take it that large areas of the earth's surface 

 have been, and probably now are, really resting upon a substratum 

 of liquid or nearly liquid rock, and that in many cases the cooler and 

 heavier surface rocks have subsided bodily, the liquid risiug through 

 great fissures and spreading over them as they sink. Here there are 

 no signs of compression ; rather has the crust been stretched to form 

 the fissures through which the lava welled up. 



No ordinary amount of compression could give rise to a mountain 

 range ; for as the rocks thickened locally under the compressive 

 forces, their weight would cause them to sink, and depression might 

 result rather than elevation. It has been suggested that the heating 

 up is caused by compression. However, in the case of North 

 America, where the vast quantities of basic lava welled up, the rocks 

 are generally horizontal and show no signs of compression. All that 

 can be made out is that mountain chains seem to have risen in areas 

 which have been ones of active recent deposition, and do not appear 

 to have formed in areas covered by rocks of great age. All our great 

 modern mountain ranges were raised in Tertiary times over areas of 

 recent deposition. 



It is probable that our anticlines and synclines are generally the 

 result of the irregular sinking of the earth's surface where it rests 

 upon fluid or very plastic magmas. The syncline of the Thames 

 Valley, for example, would seem to have varied from time to time, 

 sometimes becoming more pronounced and then flattening out again. 



The view that rocks of varying density resting upon a liquid 

 substratum may produce synclines and anticlines has already been 

 suggested by Coleman.^ He says: "Some years ago I ventured 

 another explanation. Granite is specifically lighter than most of the 

 greenstones and schists of the Kewatin ; and molten granite, even if 

 not at a very high temperature, is lighter than the relatively cold 

 rocks above it. If the rocks above were unequally struck, so that 

 some areas were less burdened than others, it is conceivable that 

 these differences in gravity might cause the granite to creep slowly 

 up beneath the parts with lightest loads, whilst overlying rocks 

 sagged into synclines in the heavily loaded parts." 



" Whatever the cause, these batholiths enclosed by meshes of 

 schist are the most constant feature of the Canadian Archaean, 

 though in many places erosion has ci;t so deeply that the meshes 

 have all but disappeared, leaving only straight or curving bands of 

 hornblende schists enclosed in Laurentian gneiss." 



Here we appear to have a mountain mass so deeply dissected by 

 denudation that the cause of the existence of the anticlines and 

 synclines which no doubt characterized the ranges is disclosed. The 

 folds were not formed by lateral pressure, but by the stretching of 

 the beds of rook as some portions settled down and others rose. 



^ Presidential Address, Brit. Ass. Eep., 1910, p. 54. 



