internal Structure of the Magnesian Limestone. 123 



described*. There can be no doubt that the magnesian hmestone has been 

 chiefly produced by mechanical agents ; and I have endeavoured to show that 

 the most crystalline portions of it have gained their present structure from 

 internal movements after the regular deposition of the formation. It is there- 

 fore probable, that a part of the magnesian earth has been derived from the 

 breaking up of the dolomitic beds in the neighbouring chain of mountain 

 limestone. The much greater proportion of dolomitic earth existing in the 

 derivative deposit admits also of explanation. The waters which filter through 

 the magnesian beds, constantly take up and bear away with them a small por- 

 tion of carbonate of lime, but reject the dolomite. Thus the stalactitic matter 

 deposited by the streams which have their source in the magnesian beds, does 

 not contain magnesia. May not the waters of the ocean, during a long suc- 

 cession of ages in which they were grinding down the carboniferous chains 

 and depositing the overlying beds on their flanks, have, in the same manner, 

 continued to take up and bear away a portion of carbonate of iime, and on 

 that account have left a larger proportion of dolomitic earth in the residuum 

 than existed in the parent formation ? 



Granting all the weight to this mechanical hypothesis which it deserves, 

 we must at the same time allow that there are some important facts of which 

 it offers no adequate explanation. In the middle of Yorksliire the magnesian 

 limestone is at a great distance from the calcareous portions of the older rocks, 

 and, I believe, contains more magnesia than could have been obtained from the 

 nearest chain of mountain limestone, though the whole of it had been broken 

 up for the supply. Again, on this theory, no part of the dolomitic series ought 

 to contain more magnesia than enters into the composition of the definite 

 triple salt. But there are some instances of beds in this series which are by 

 no means of crystalline texture, and contain not less than sixty per cent of 

 carbonate of magnesia f. 



happens to contain certain fragments of trap and porphyry. These appearances are the exception 

 and not the rule ; on which account I think that the Heavitree conglomerate sliould never be 

 given as the type of one of the English formations. It forms a rare variety, and nothing more. 



In Germany, trap rocks are extensively associated with formations of the same age with those 

 described above. The great importance which is given to them in determining the epoch of 

 certain deposits can, however, only be regarded as a lingering remnant of that false theory which 

 considered trap rocks of the same origin with regular secondary strata. In England, with certain 

 exceptions, trap rocks are not associated with deposits of the age described in the tvxt. Here 

 there is a broad lino of distinction, and one which might have been expected a priori: and, if it 

 prove any thing, it proves that trap rocks cannot much assist us in identifying the regular de. 

 posits of distant regions of the earth, and ought never to be used among the prominent types of 

 comparison. 



* For some beautiful observations on this subject, see Geol. Trans. Second Series, vol. i. 

 p. 291, kc. 



t See p. 105, note. 



r'2 



