1882.] 79 [Crosby. 



quakes as the cause of jointing, since the joints are incidental to 

 the formation of the fissures represented by veins, dikes and 

 faults. 



Again, joints are not only arranged in sets by their parallelism, 

 but the sets cross each other at all angles without interference. 

 The same is undoubtedly true of earthquake-fractures. An earth- 

 quake, of course, moves in all directions, but the area affected is 

 usually so great that in any limited tract, not too near the epicen- 

 trum, it will seem to move in only one direction. A series of 

 vibrations travelling from east to west would produce a set of 

 parallel joints running north and south ; and these will be no 

 obstacle whatever to vibrations moving in a north-south direction, 

 which would give rise to a set of east-west joints, crossing the 

 first set at right angles and absolutely without interference. And, 

 similarly, a shock coming from the north-east, or some other inter- 

 mediate point of the compass, would determine a set of joints 

 crossing both the preceding sets obliquely, and so on. 



It has been suggested to me by Professor Hitchcock and several 

 other persons, as an objection to the earthquake theory of jointing, 

 that among the blocks produced by jointing it may be observed 

 that certain forms are more characteristic of some rocks than of 

 others, rhomboidal blocks, for instance, being especially com- 

 mon with slate and cubical blocks with sandstone and coal. Proba- 

 bly all geologists have observed something of this sort ; but it is 

 easy to exaggerate such distinctions, and I believe that they are 

 largely fanciful and that in so far as they actually exist they may 

 be satisfactorily explained without surrendering the theory here 

 advocated. There is a vast difference in the smoothness and 

 regularity of the blocks in different kinds of rocks. To appreciate 

 this we only need to compare the compact sedimentary rocks like 

 slate and limestone with the coarse-grained and crystalline rocks 

 like granite, etc. But these differences, it is clear, are determined 

 by differences in the texture and elasticity of the rocks, and are 

 no argument whatever against the view that the fractures are due 

 to vibratory movements of the rocks , for the same series of vibra- 

 tions would inevitably produce smooth and regular fractures in 

 slate and exceedingly rough and irregular fractures in granite. 

 It is unquestionably true that approximately rectangular blocks 

 are far more common in all kinds of rocks than blocks with very 



