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XX. An Examination of those Phenomena of Geology, which 

 seem to bear most directly on theoretical Speculations. By 

 the Ecv. W. D. CONYBEARE, M.A. F.R.S. F.G.S. $c. 



[Continued from page 23.] 



VIII. r pHE distribution of the rocks usually considered as 

 - of volcanic origin, in the different formations, is 

 such as to indicate the greater prevalence of volcanic agency 

 during the earlier periods ; and the relations of the actual vol- 

 canic vents are such as to prove that these are only the resi- 

 dual portion of a much larger number which have coexisted 

 in the antecedent epochs. 



Observations. This article ought strictly to have occupied 

 an earlier place in my arrangement, and to have immediately 

 followed No. IV. but as it will be seen that many causes tend to 

 throw obscurity on this part of our subject, and to prevent 

 our arriving at more than approximate results, I have judged 

 this departure from a more exact method justifiable, as I have 

 thus been enabled to give precedence to the evidence which ap- 

 peared to me most clear and satisfactory. From the intrusive 

 position of these rocks, which appear very frequently to have 

 been injected among the strata which they traverse, subse- 

 quently to the deposition of those strata, a difficulty arises, in 

 limine, as to the determination of their age. We know them to 

 be subsequent to the beds traversed ; but who shall say how 

 much so? In order to ascertain this point, the junction of 

 these beds with the succeeding formations should be carefully 

 examined, and the exact geological point noted where these 

 intruding masses are cut off and cease to traverse those for- 

 mations. For instance, if a trap dyke shall be found travers- 

 ing the coal measures, but cut off by the incumbent mag^ 

 nesian limestone, we may then be sure that the cause which 

 produced this dyke was in action before the deposition of the 

 magnesian limestone. We have, however, hitherto few obser- 

 vations to this effect; but it yet seems to me that an approxi- 

 mation is attainable ; for we find that the rocks usually con- 

 sidered as igneous, materially vary in their characters in the 

 different^formations ; granite occurring most generally in asso- 

 ciation with the rocks termed primitive; peculiar greenstones 

 and porphyries with those of transition ; others again with 

 some varieties of basalt in the coal formation. Now whenever 

 we find peculiar varieties always associated with a single for- 

 mation, and excluded from the contiguous formations of more 

 recent date, we may fairly, I think, infer that their production 

 has taken place almost contemporaneously with that of the 

 formations in which they so occur ; yet it must be owned that 



the 



