TITLES AND ABSTRACTS OF PAPERS 85 



which the old topographic surface is intersected by the fault-scarp. The up- 

 ward movement may be described as a high angle upthrust. The plane of the 

 thrust rises to vertical or even beyond vertical and appears, especially when 

 eroded, as a normal fault-plane, hading to the downthrow. In fact, however, 

 it is by hypothesis neither normal nor plane. It is regarded as a curved thrust 

 surface, curving back under the block. 



Since the under side of the block can not be observed, its form is hypothet- 

 ical ; but if the rotated block moved on it as on a guiding surface, it must be 

 a curved surface, as described above. The only alternative would be that the 

 mountain block floats. This is dismissed as inadmissible. Hence the curved 

 under surface of the block is regarded as proved by the rotation. 



The under surface of the block is extensive. Its visible margin is the fault- 

 scarp. It extends downward probably many miles and seaward many tens of 

 miles. It is a major structure which requires explanation. 



Independent studies i»to the possible effects of isostatic adjustment had led 

 the writer to conclude that erosion and sedimentation are quantitatively too 

 weak to produce mass movements in the rigid earth, but that they do set up 

 stresses adequate to direct the orientation of crystals in the development of 

 schists. The orientation should vary from a horizontal attitude of foliation 

 beneath loaded areas to a vertical attitude beneath lightened areas, and should 

 constitute a curve rising from a great depth beneath the ocean basins to the 

 continental border zones. Shear stresses at approximately 45 to the horizon- 

 tal should supplement the effects of orientation by isostatic stress. Long time 

 is a very important factor. The result should be a discoidal structure of the 

 lithosphere. It is believed that this structure is a general condition to a depth 

 of a hundred miles or more. The hypothesis has been tested as a clue to 

 understanding various phenomena of orogeny, vulcanism, and mineral zones. 

 In connection with the rotated mountain blocks of California, it is held that 

 their under surfaces are elements of the discoidal structure of the lithosphere. 



The force which moves the rotated mountain blocks and causes them to rise 

 on their guiding under surfaces is found in the weight of the failing sub- 

 oceanic mass, as when water pushes out a dam. In discussions of isostatic 

 balance it is commonly assumed that the opposed horizontal pressures of two 

 adjacent failing columns are transmitted through homogeneous or structure- 

 less material. To produce motion their difference would then needs be greater 

 than the crushing strength of the material. But when an inclined foliation 

 ha% developed, the components of pressure against an inclined plane come into 

 play and cohesion is largely replaced by friction as a resistance to upthrust 

 along that plane. Even so, it is the fact that stability of the lithosphere is 

 the prevailing condition. Imperceptible warping has been more common than 

 energetic mountain growth. To explain the latter, hypothesis appeals to the 

 occasional greater activity of recrystallization and of melting in deep-seated 

 schists, due to variations of internal heat. The two processes may or may 

 not be associated. Recrystallization may produce gradual movements. Melt- 

 ing and the intrusion of molten sheets along planes of schistosity may so re- 

 duce friction as to occasion revolutionary orogenic disturbances. Some evi- 

 dence of association of igneoius rocks with orogeny can be adduced, but in 

 general the relation remains hypothetical because the zone of activity is be- 

 yond observation. 



