PRINCIPLES OF GEOLOGY. 15 



is filled with mixed fragments of stone ; in dykes, by basalt or other 

 rocks ; but in mineral veins, with sparry and metallic minerals. 



The convulsions within the earth which have thus changed the in- 

 clination, altered the position, and broken the continuity of rocks in 

 so remarkable a manner, happened, of course, since the deposition and 

 induration of all the strata which have been thus dislocated. But such 

 progress has been made in inductive geology, as to render it evident that 

 some of these irregular operations had been completed in certain strata 

 before the next rocks were deposited upon them. For in Somersetshire, 

 coal measures, highly inclined, lie beneath and are concealed by horizon- 

 tal beds of red marl. Therefore, their highly sloping position must have 

 been determined before the deposition of that rock : and in the same 

 country great faults, which elevate the coal seams seventy yards, pro- 

 duce not one inch of displacement in the red marl which lies above. 

 Assuredly, then, internal convulsions of the earth occurred at intervals 

 during the deposition of rocks ; and by studying their relative antiquity, 

 we obtain plain evidence of the lapse of time between the formation of 

 the several strata. Examples of the same kind are well known in York- 

 shire, where inclined coal measures are covered (as at Garforth) by nearly 

 horizontal magnesian limestone, which is unbroken by the vast dykes 

 in the subjacent coal. There are good grounds for believing that the 

 highly-inclined position of the primary strata is not original : it is ex- 

 tremely probable, and, indeed, generally admitted, that these stupendous 

 ranges of mountains have been uplifted by some mighty internal agency. 

 It is certainly true that the greatest dislocations of the secondary strata 

 are in the vicinity of primary mountains ; and, though it must not pass 

 as a general or established rule, we may sometimes refer the disruption 

 of secondary to the same agency which produced the elevation of pri- 

 mary rocks. 



HAVING considered the internal structure of our planet, and shewn 

 how the rocks succeed one another in a fixed order, and rise successively 

 to the surface ; how variously they are filled with the monumental re- 



