372 



DYNAMICAL GEOLOGY, 



346. 



345. 



the reflections from the surfaces of fracture give a false appearance of ridges 

 along the fractures.) Daubree draws attention to (1) the 

 approximate parallelism of the lines, and yet their slight 

 divergence ; (2) the crossing of one set of lines by an- 

 other nearly at right angles, anti-jKirallels, as he calls 

 them ; (3) the fact that the lines are in groups ; (4) the 



fact that joints may be 

 an instantaneous effect ; 

 (5) the very important 

 fact that the force pro- 

 ducing the joints did not 

 act at right angles to 

 either set, but at the 

 extremity of a bisectrix 

 to the angle of intersec- 

 tion of the two sets; 

 and (6) the fact that 

 the slower the action 

 of the force and the 

 larger the plates, the 

 nearer the ap|)roach to 

 parallelism between the 

 lines in each set. Fract- 

 ures made by torsion 

 might be left open when 

 those from direct pres- 

 sure would remain 

 closed. Other instruc- 

 tive figures are given in 

 his work on Experimen- 

 tal Geology. Joints may also be due to the vibrations of earthquakes 

 (Crosby), and to changes of temperature (pages 260, 264). 



J'^ 



w. 



Lines of fracture produced In a plate of ice 

 (G-G) by slight torsion, (x J,.) 



Portiou of a plate of 

 ice showing its fract- 

 ures (x^). From a 

 photograph. 



2. Earthquakes. 



An earthquake is a series of vibrations begun in some region of local dis- 

 turbance in the earth's crust, and propagated upward and outward from this 

 place as a center. Slight tremors may be produced by falls of large rock- 

 masses, where undermining has been carried on. But true earthquakes 

 come, for the most part at least, from one or the other of the following 

 sources of disturbance : — 



(1) Vapors suddenly produced, causing ruptures and friction; or, com- 

 monly, (2) sudden movements or slips along old or new fractures. 



Earthquakes due to the former of these methods are common about vol- 

 canoes. At the Hawaiian Islands, shakings that are destructive over the 

 island of Hawaii at the moment of some of the more violent eruptions do 



