802 



DYNAMICAL GEOLOGY. 



adjoining the fracture, as a consequence of the molecular movements 

 in the bulging, fine rectangular cracks were produced, like a delicate 

 network. 



Cubes of hard rock usually break off at the angles and edges, leav- 

 ing two rounded cones with their tops at the middle ; but a tabular 

 block of limestone under vertical pressure was reduced by Daubree to 

 vertical prisms and plates. 



Daubree has also beautifully illustrated the production of a system 

 of joints by a slight torsion. He subjected plates of ice to this action 



and obtained, as one of his results, 

 Fig. 1157. with a plate nearly a yard long, 



the fractures shown on a much re- 

 duced scale in Fig. 1157. Figure 

 1158 shows a portion of one of 

 the plates one fourth of the natu- 

 ral size. [It is from a photo- 

 graph, and hence 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- 

 parallels, 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 producing the joints did 

 not act at right angles to either 

 set, but at the extremity of a bi- 

 sectrix 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 approach to parallelism 

 between the lines in each set. Other instructive figures are given in 

 his important work on Experimental Geology.' 



' It is apparent from the statements on page 789 that some amount 

 of torsion has generally taken place in connection with the folding and 

 fracturing of rocks. The lines of trap dikes in the Connecticut val- 

 ley (map, p. 19), probably owe something to it. It is a consequence 

 of fracturing by torsion that the fractures it produces might be left as 



Lines of Fracture in a Plate of Ice produced by 

 slight torsion, (x -j 1 ^.) 



