Crosby.] 80 [October 4, 



oblique angles. The proper explanation here, I think, is that, 

 after the rocks have been broken by one set of joints, the layers 

 or sheets thus formed possess a strong natural tendency to break 

 at right angles ; and, as previously stated, under such circum- 

 stances, oblique vibrations may give rise to rectangular fractures 

 and blocks. 



Granting, for the sake of the argument, that, as Professor 

 Hitchcock and others claim and as I deny, aside from the vary- 

 ing degrees of smoothness and regularity of the joint-blocks, 

 certain general forms, as rhombohedrons or cubes, are peculiar to 

 particular kinds of rock, still it must be admitted that this correla- 

 tion is as easily harmonized with the earthquake as with the 

 shrinkage hypothesis, for it appears, in fact, to be explainable by no 

 hypothesis, save the absolutely baseless one that jointing is a 

 species of crystallization. 



Prof. James Hall hns called my attention to the fact that the 

 Post-tertiary clays in the vicinity of Albany, N. Y., like those of 

 the Great Salt Lake Desert, possess a well developed joint- 

 structure, two sets of joints crossing at right angles. And he 

 has expressed the belief that this is an example of parallel joint- 

 ing which can not be due to earthquake action, because of the 

 inelastic nature of the clay and the fact that it rests upon a bed 

 of gravel, which, it is thought, protects it from all breaking shocks. 

 But, if the stratum of gravel affords this measure of protection to 

 the clay, it becomes difficult to understand how areas underlaid 

 by gravel can ever be disastrously affected by earthquakes, as 

 they undoubtedly have been. A large part of the earth's surface 

 has essentially the same structure, consisting of alternating beds 

 of gravel and clay. Besides, it is perhaps possible, that at Albany, 

 the shocks, travelling horizontally, might pass " end on " into the 

 clay without traversing the gravel. The inferior elasticity of the 

 clay, so far from being an argument against the earthquake 

 hypothesis, would seem to tell strongly in its favor, since the prob- 

 ability that vibrations will be attended by fractures, is inversely 

 proportional to the elasticity of the medium. In other words, we 

 might safely grant that only slight vibrations can enter the clay, 

 because these would be sufficient to break a medium so inelastic. 

 Professor Hall refers the jointing of the Albany clays to the 



