Ch, XVI.] OF THE PREVAILING THEORY. 389 



effect of violent injections and protrusions, the other is sup- 

 posed to have arisen from the gradual refrigeration of a fluid 

 mass : and, besides, we have not been able to detect in the 

 primary rocks any indication of such occurrences. We will, 

 therefore, as during the consideration of atmospheric and 

 aqueous causes, begin at the epoch when the formation of the 

 oldest detrital deposits commenced. From that time, different 

 parts of the surface of the earth have experienced successive 

 alterations in the relative levels of sea and land, which may 

 be fairly referred to the expansive force of the internal fire, 

 since similar phenomena have been observed in volcanic 

 countries. During each geological epoch, volcanos appear 

 also to have existed : and that they communicate with or even 

 below the primary rocks, is rendered very probable by the 

 ejection of fragments of such rocks ; by their cones in some 

 instances reposing on granite; and, lastly, by the com- 

 position of lava, trap, basalt, and similar substances, which 

 contain a portion of alkali, and which may therefore 

 have been derived from the same source as the primary 

 rocks. Lastly, each sedimentary group is traversed by dykes 

 and various masses of crystalline and alkaliferous rocks, which 

 possess all the characters of an igneous origin : these rocks 

 in the oldest deposits often resemble the primary, both in 

 composition and in external and internal properties, and can 

 only be distinguished therefrom by their geological position 

 and mode of union with the adjacent schistose rocks. Whe- 

 ther such dykes or intrusive igneous rocks do actually occur 

 in the primary at great depths, or whether they have traversed 

 every part thereof during each sedimentary epoch, is a ques- 

 tion which our present knowledge does not enable us to 

 determine. The primary igneous rocks may be so cir- 

 cumstanced with respect to the intrusive ones ; and such a 

 condition would not be incompatible with the hypothesis 

 which considers granite, and its associated schists, to have 

 been contemporaneously formed before the existence of the 



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