Crosby.] 82 [October 4, 



to a powerful and sudden jerk. No argument is required to 

 prove that, other things being equal, a strain will produce a more 

 regular fracture if quickly than if slowly developed. 



My purpose now is to show that in certain rocks which, under 

 ordinary circumstances, break very irregularly, and yet possess a 

 joint-structure of remarkable straightness and regularity, earth- 

 quakes are a far more probable cause of the jointing than either 

 shrinkage or compression. As a magnificent example of such a 

 formation, I would refer to the conglomerate so extensively 

 developed in the vicinity of Boston. Few rocks are more hetero- 

 geneous in texture and composition, and the artificial fracture is 

 exceedingly uneven, rarely dividing the pebbles and almost never 

 dividing them smoothly. But the joints, of which there are 

 several w T ell defined sets, are beautifully, almost mathematically, 

 regular — true planes, passing through pebbles and cement alike, 

 as by one clean, swift stroke. It is rare to find a case where the 

 joint is deflected by a pebble ; but very commonly the divided 

 pebbles are slightly faulted, the two parts no longer correspond- 

 ing in position. The dislocation varies from a small fraction of 

 an inch, to a foot or more. The pebbles are chiefly quartzite, 

 petrosilex and granite, and are distinctly stronger than the cement. 

 Under the influence of shrinkage they would be dragged from 

 the cement along the fractures, and neatly divided pebbles w T ould 

 be the rare exception, instead of the nearly universal rule. The 

 conglomerate at Newport, R. I., affords another good example of 

 wonderfully regular parallel joints in a rock, the coarseness and 

 irregularity of which are rarely exceeded. 



In my paper already referred to in the Geological Magazine, I 

 have taken for granted (as geologists have heretofore been accus- 

 tomed to do) that, although joints are sometimes due to move- 

 ments of the rocks, contraction is the more general and efficient 

 cause. And, proceeding upon this assumption, I have there 

 endeavored to show that, as a rule, during the deposition of sed- 

 iments and their subsequent burial beneath newer formations, the 

 ordinary causes of contraction — consolidation and crystallization 

 — will be neutralized by the increased temperature of the sedi- 

 ments ; and that it is only when these are brought back to or 

 toward the surface by erosion, that a net contraction can take 



