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CHAPTER XVI. 

 HISTORY OF BRITISH PLUTONIC ROCKS. 



Age of Igneous Rocks. When in any country a certain class of 

 rocks, as, for instance, the slate rocks, has been convulsed and thrown 

 into new positions before the deposition of another series upon them, 

 as, for instance, the carboniferous rocks, and we find a mass of 

 granite occupying the axis or nucleus of the dislocation, it is certain 

 that such granite is older than the carboniferous system, because it 

 was uplifted with the older slates. If, in addition, this granite sends 

 veins through the slate rocks so as to prove that it was uplifted in a 

 melted state, we must infer that it is (considered as a solid) of more 

 recent origin than those slates ; and, in fact, that the antiquity of its 

 latest fusion is exactly measured by the date of the convulsion. 



If there be no veins thrown off from the mass of granite, and 

 no other satisfactory proof of its having been uplifted in a melted 

 state, the age of the igneous rock is indefinable, except by saying 

 that it is older than a given stratified rock. Such a case occurs in 

 the Ord of Caithness. It appears, then, that in any case of convul- 

 sion, the era of the elevation of the igneous rock is determined by the 

 convulsion, but whether the rock was actually generated at that time 

 from a melted state requires other evidence. Now this consolida- 

 tion from a melted state is what fixes the age of an igneous rock. 

 Granite may perhaps have remained in the deep parts of the earth 

 at a melting temperature through many geological periods, but its age 

 as a rock is counted from the period when its fusion ceased. 



In Derbyshire the carboniferous limestone is interlaminated for 

 great lengths with a basaltic rock (toadstone), which has evidently 

 been poured out at certain intervals by an ancient submarine volcano 

 while the limestone was in formation. The age of such a rock is 

 fixed by the age of the limestone. 



The basalt of dykes which pass through certain strata is, of 

 course, not more ancient than the newest strata divided ; if at 

 any point the dyke should be covered by newer strata which are 

 undisturbed by the dislocation accompanying it, wo may generally 

 admit that the basalt is older than these strata. Such a case, 

 perhaps, occurs in those dikes of the Durham coal-field, which 

 do not penetrate the magnesian limestone; but it is not always 



