1882.] 81 [Crosby. 



shrinkage-hypothesis, although, as Mr. Gilbert has shown, it 

 possesses almost none of the characteristics of true shrinkage- 

 cracks, of cracks known to be due to shrinkage ; while it is diffi- 

 cult to see wherein the earthquake hypothesis falls short of a 

 perfect explanation of the phenomena. 



Apparently, there is no important characteristic of parallel 

 jointing, which is not satisfactorily explained by a reference to 

 earthquakes as a cause. A moment's reflection, however, will 

 show that we can not properly regard earthquakes as the sole 

 cause, but simply as a principal cause, of this class of joints. The 

 phenomena of elevation and plication must be accompanied by 

 fractures ; and these, it is easy to see, will have the normal fea- 

 tures of ordinary or parallel joints, being parallel, and intersect- 

 ing without interference when the strains are varied in direction. 

 But we must not lose sight of the fact that earthquakes account 

 for joints in rocks of all classes and conditions — in the uncon- 

 solidated and undisturbed post-Tertiary clays of the Great Salt 

 Lake Desert, as well as in the ancient crystalline and contorted 

 gneisses ; while the normal accompaniment of compression and 

 plication, on the other hand, is cleavage, and not jointing; al- 

 though neither structure can be developed by these agencies 

 without disturbing the stratification. 



With shrinkage and compression, time is an important element. 

 The strain is developed very gradually, and a well marked frac- 

 ture, due to these causes, may be days or years in forming. The 

 earthquake, on the other hand, is just the reverse. According to 

 Mallett, the amplitude of the earth-waves varies from three or 

 four inches to as many feet, and the velocity of vibration, or of 

 shock, from ten or twelve feet to eighty feet per second, giving 

 an average of about fifteen complete vibrations per second. In 

 other words, the rocks, during the passage of the earthquake, are 

 jerked violently through a distance of several inches or feet and 

 back again fifteen times in a second, or at that rate. The energy 

 displayed is immense, but its application is so nearly instantaneous 

 that the element of time almost disappears. Compression and 

 shrinkage mean a very gradual pressure or an equally gradual 

 pull ; but in the earthquake the push and pull are combined, the 

 first being like a heavy sharp blow, while the latter is equivalent 



PROCEEDINGS B. S. N. H. VOL. XXII. 6 APRIL, 1883. 



