58 TEANSACTIONS OF THE [oCT. 18, 



metamorpliism which are distinctive features of all mountain 

 belts and chains. Where this action takes place on a grand 

 scale, and involves the entire thickness of the earth's crust, 

 mountain chains of great elevation, length, and breadth are the 

 result, and, through profound fissures opened to the zone of 

 molten material below, lava rises and overflows. In theTriassic 

 age, such deep fissures were opened along the Atlantic border of 

 the continent, as is attested by the trap sheets and dikes which 

 extend interruptedly from Nova Scotia to the Carolinas. At 

 that time, the whole coast must have been shaken with earth- 

 quakes of great violence, and much of it devastated by lava- 

 floods. Since then, the eartii's movements have been only the 

 relatively gentle vibrations caused by the yielding of the flexed 

 and fractured rocks of the upper portion of tlie crust to the 

 ever acting and resistless thrust of the great unbroken tables of 

 the Mississippi Valley and the Atlantic basin. 



EAETHQLTAKES AKD VOLCANOES AS MEASURES OF THE THICK- 

 NESS OF THE earth's CRUST. 



As is mentioned in the early part of this article, the first re- 

 sult of the discovery of the law of increase of temperature in 

 ^oing toward the centre of the earth was the conclusion that the 

 solid crust was not more than 50 miles in thickness, and below 

 that was a sea of fluid or semifluid molten matter. Then moun- 

 tain chains were supposed to be the result of the crushing to- 

 gether of solid sheets of rock as they followed the cooling and 

 shrinking interior. The coat becoming too large, and adhering 

 to the body, must wrinkle as the body shrank. Volcanic erup- 

 tions were supposed to be the oozing out of molten matter from 

 the not distant zone of fused material, and all was harmonious 

 in the geological world. Then came Professor Hopkins, Arch- 

 deacon Pratt, and Sir William Thomson, in the character of dis- 

 turbers of the public peace; they said that the crust would be 

 broken up by tides if it was as thin as supposed; that the shell 

 would be pulled about on the fluid nucleus by the attraction of 

 the moon on the equatorial protuberance; and, finally, that the 

 tenacity with which the figure of the earth is maintained under 

 the varying pull of the sun and moon made it necessary to sup- 

 pose that it was, as a whole, as rigid as a globe of glass, or even 

 of steel. Sir William Thomson conceded, with some hesitation, 

 that the crust of the earth might not be more than 2,500 miles 

 in thickness; further than that he would not go. Since that 

 time, awed by his great and well-deserved fame, geologists have 

 generally accepted the conditions he imposed upon them, and 

 there has been a terrible struggle to reconcile volcanoes, earth- 



