R. M. Deeley — Mountain Building. 117 



a slope under the action of gravity, every yard of the mass furnishing 

 its own propulsive force. 



How the dense "roots" of depressed areas and the lighter 

 "roots" of elevated areas arise, it is not my intention to discuss. 

 That such roots exist a perusal of the paper by K. D. Oldham 

 already referred to will demonstrate. 



Rain, rivers, etc., attack the elevated areas, cutting out river 

 valleys of varying width. The character of the resulting scenery 

 depends upon the nature and hardness of the rocks. The depth and 

 width of the valleys resulting from denudation must bear a close 

 relationship to the forces required to make the rocks flow. When 

 the rocks are soft and erosion results in the production of deep 

 valleys, then the valley bottoms rise, the sides close in, and various 

 irregularities in the bedding planes result. In this way anticlines 

 and synclines are very frequently formed loithout lateral pressure. 

 Such movements are greatly assisted by temperatures. 



The crust of the earth grows warmer and warmer as the depth 

 increases. This gradient of temperature in the crust is about 1° F. 

 for every 70 feet of descent. At great depths the rocks are con- 

 sequently very hot. Now a solid almost always becomes softer and 

 softer as the temperature is raised, and the force which will cause 

 it to flow becomes smaller and smaller. If the tempei'ature were 

 raised so high that the force which would produce continuous flow 

 fell to almost zero, then the solid would have been abruptly turned 

 into a liquid, and flow would result however small tlie force might be. 



When a plastic substance is distorted microscopic examination 

 shows that it has resulted from the formation of many distinct shear 

 planes. On the other hand, the distortion of a viscous substance is 

 caused by shear between planes whose thicknesses are of molecular 

 dimensions. 



A piece of granite overloaded would break into small fragments. 

 It would not flow like soft clay. However, if the granite were 

 surrounded by some substance under very great pressure, then the 

 particles of rock would be so firmly held together that it would not 

 fracture if deformed; but would change its shape without breaking. 

 It thus comes about that althougli hard rocks are fractured by earth 

 movements near the earth's surface, they remain massive at great 

 depths although they sufFer distortion. This ability of rocks at great 

 depths to suffer distortion without fracture is, of course, much 

 assisted by the high temperature prevailing there. 



The idea that the surface rocks are floating in very many places 

 upon a fluid stratum can be shown to be probable by many 

 phenomena. During Tertiarj^ times, over an area of some 200,000 

 square miles of what is now Idaho, Oregon, and Washington, fluid 

 basic lavas welled up more or less steadily through great fissures, 

 and covered the whole country with sheets of lava aggregating in 

 places 2,000 feet or more in thickness. In India a like area was 

 covered to a depth of from 4,000 to 6,000 feet, while over the Lake 

 Superior basin and some of the surrounding areas the thickness 

 reached by more ancient lava floods was from 15,000 to 25,000 feet. 

 There are amongst these lavas very few beds of ash or cinders, so 



