Murchison! s Silurian System. 49 



eruptions to the extent to which they must have existed in 

 the early history of the planet could not have occurred 

 without producing striking changes in physical geography. 



The volcano and the earthquake are said by Mr. Murchi- 

 son to be, the one a " safety valve" by which heated matter 

 escapes at intervals from the interior, the other is the shock 

 which lacerates the earth when the heated matter and its 

 vapour is denied an access, and the task of the geologist is to 

 read off the proofs of successive eruptions amidst the ruins they 

 occasion.* Mr. Murchison then alludes to the observations 

 of Hutton, Playfair, Hall, and others, to prove that syenites, 

 porphyries, green stone, clink stone, and basalts are of 

 igneous origin, but as this is now pretty generally allowed, 

 we may pass over this part of the work. Mr. Murchison how- 

 ever adduces numerous circumstances, the result of his own 

 researches, to prove that the basalts which have overflowed 

 and dislocated the coal-fields must have been erupted sub- 

 sequently to the period of the New Red Sandstone, while 

 other great epochs of disturbance took place anterior to the 

 deposit of the Old Red and Carboniferous rocks. The 

 types of the Silurian system and the associated volcanic 



* To afford an illustration of the manner in which geologists estimate the anti- 

 quity of rocks ; let us suppose that fishes were buried in the ruins or debris of 

 Graham's island, when the light materials of which it was composed, were scat- 

 tered over the bottom of the sea, by the violence of currents. If by some future 

 convulsion, that part of the sea should be raised up so as to compose dry land, and 

 so many ages to elapse as to destroy all record of the change, except such as the 

 remains of fishes imbedded in the rocks would afford, of the latter having been form- 

 ed beneath the sea ; the future geologist would then institute a comparison between 

 the remains of the fishes, and such species as might live in his day ; if he found 

 them correspond, he would conclude the change from sea to land to have been 

 comparatively recent, but if many of the fossils presented the characters of 

 species unknown in his day as inhabitants of any part of the globe, he would con- 

 clude that they belonged to forms that have become extinct, and from all he could 

 gather regarding the period of duration assigned to species, he would form his 

 calculations as to the period in the earth's history at which Graham's island was 

 overthrown. — En. 



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