9o6 PRINCIPLES OF STRATIGRAPHY 



greatest. In any case, folding is shown to be a superficial phenome- 

 non, a conclusion which must have an important bearing on the 

 mechanics of folding. As shown by the section, the eastern portion 

 suffered most, the western portion was the next most affected, while 

 the intermediate ones suffered the least. The form of the entire 

 block affected is that of a triangular prism; the lower bounding 

 planes being shear-zones. This is the form which such a block 

 should assume theoretically, two sets of planes of greatest tangen- 

 tial stress developing at right angles to each other, and approxi- 

 mately at an angle of 45 degrees to the direction of greatest pres- 

 sure, though varying with the nature of the material and other 

 factors. (Becker-i 150; Hoskins-20 i^'dj ; Leith-27 i/i"/.) Experi- 

 ments by Daubree have shown this to be the type of fracture de- 

 veloped in compressed blocks where a triangular prism is developed 

 with faces approximately at 45 degrees to the direction of pressure. 

 In the experiment the prism was lifted without folding, the other 

 parts being thrust under it on both sides (Fig. 251). The exten- 

 sive development of over (or under) thrusts along the western 

 margin of the folded Appalachian region is quite in harmony with 

 this principle. It should, however, be mentioned here that there 

 is some evidence that the folds of the Appalachians were not purely 

 asymmetrical anticlines, modified by overthrust, but that the Ap- 

 palachians had originally a fanfold structure. 



Changes Due to Extra-Telluric Influences. 



The impact of a mass of meteoric matter upon the surface of 

 the earth would form an effective source of energy, which would 

 set into motion geological agents of vast magnitude. If the mass is 

 sufficiently large, as in the case of a satellite, a great displace- 

 ment of the earth's center of gravity would result, with the 

 attendant phenomena already outlined. If the earth v.^ere bom- 

 barded by a vast number of meteoric bodies, if. in other words, 

 a swarm of meteors should descend upon the earth, changes affect- 

 ing the entire earth might take place. Among these might be the all 

 but universal extermination of life, as well as the modification 

 of the lithosphere and hydrosphere, and perhaps the atmosphere 

 as well. The heat of impact might result in nearly universal meta- 

 morphism of the rock masses of the earth, with the accompanying 

 destruction of the record of life in these rocks. It might be 

 asked if some such catastropiie may not have altered the surface 

 of the earth in pre-Cambric time, inaugurating the forces which 



