I8S2.J S3 [Crosby. 



place, and joint-structure be developed; theiinmediatecau.se of 

 the contraction and jointing being, not consolidation or crystalll* 

 nation, but cooling. 



Contraction in the vertical direction in sediments must always 

 be inoperative, so far as the formation of fractures is concerned. 

 And my father, F. W. Crosby, has; recently suggested to me 

 another reason why vertical shrinkage-cracks or joints cannot 

 appear in deeply buried rocks, visfc t that the enormous pressure of 

 the overlying strata would cooperate with the heat in causing 

 lateral expansion and preventing contraction. The pressure at a 

 depth of only one mile must be nearly one million pounds on 

 every square foot, sufficient to cause all ordinary rocks to spread 

 horizontally, and neutralize any conceivable contraction. 



In support of the theory thus worked out, I have brought the 

 testimony of the coarsely crystalline eruptive rocks. These 

 masses, as most geologists agree, have been formed at considerable 

 depths in the earth's crust, appearing on the surface only as the 

 result of immense erosion ; and yet their outlines are usually 

 extremely irregular and ragged, showing that,, at the time of their 

 intrusion the enclosing rocks must have been essentially un jointed. 

 My aim, in this connection, is to show that the argument for the 

 superficial nature of joint structure is strengthened,, rather than 

 weakened, when we substitute earthquakes for shrinkage as the 

 principal cause of jointing. 



In the first place, earthquakes are comparatively superficial.. 

 This is one of the most important and firmly established proposi- 

 tions yet added to the theory of earthquake phenomena. More- 

 over, it seems pertinent to call attention here to a point which 

 has, apparently, been neglected in determining the depth of earth- 

 quake foci, but which has, nevertheless, an important bearing 

 upon that problem. Briefly stated, it is this : The rocks com- 

 posing the earth's crust, it is- universally conceded, are less 

 thoroughly consolidated, less crystalline, and more broken by- 

 cleavage, faults, etc., in other words, are far less elastic, near the 

 surface than at considerable depths. Consequently, while the 

 vibratory movement which we call an earthquake moves down- 

 ward and horizontally with comparative freedom, its progress 

 upward will be greatly retarded by the inelastic nature of the rocks 



