131 



portant to our inquiries not to be mentioned here. Some fear, 

 perhaps, may be entertained, that his data are not yet sufficiently 

 extensive to form an adequate base for his deductions ; but there 

 can be no question as to many of his inferences, nor respecting the 

 impulse which the subject will receive from such an accumulation 

 of facts as he has brought together. His views contrasting the 

 climate of the globe at former periods and at the present time, — 

 and his division of the epochs of geological deposition, as deduced 

 from the study of fossil plants, in comparison with those which mere 

 geological inquiry points out, — are most ingenious. Even if re- 

 garded as no more than the conjectures of so acute and indefa- 

 tigable an inquirer, these speculations would be well deserving of 

 attention ; and altogether, his works on Fossil Plants must be con- 

 sidered as constituting one of the most valuable contributions to 

 this department of Geology that has ever appeared. 



The Paper of Messrs. Oeynhausen and Dechen, on the structure 

 of Ben-Nevis in Scotland ; and that of Messrs. Lyell and Murchison 

 on the formation of valleys in Central France, give rise to some 

 general reflections of great interest to theory : and though the tracts, 

 in these two cases, are altogether different in geological character, 

 the inferences derived from them, combine remarkably to support 

 the opinions which at present prevail. 



The summit of Ben Nevis, the highest mountain in Scotland, con- 

 sists of porphyry ; the flanks are granite, on which again is incum- 

 bent mica-slate. Messrs. Oeynhausen and Dechen have ascertained 

 that the porphyry, instead of being an overlying mass, as has been 

 asserted in similar cases, comes up through the granite ; and that, 

 as veins shooting from the granite are found to penetrate the in- 

 cumbent mica-slate, so veins of the porphyry shoot into the granite 

 itself, and thus demonstrate the more recent protrusion of the 

 former compound. It has long been known, that granite, in the 

 Isle of Arran and at Newry in Ireland, is traversed by veins of 

 pitchstone, which itself is only a variety of porphyry : and Mr. 

 Knox's detection of bitumen in pitchstone of every age, as well in 

 various other rocks of the trap formation*, coincides with this 

 evidence, in demonstrating the igneous origin of that entire series 

 of compounds. The light which the observations of Messrs. Oeyn- 

 hausen and Dechen throw upon the " Elvans" or porphyritic 

 veins of Cornwall, was alluded to in the conversation which followed 

 the reading of their Paper here ; for these Elvans are in fact great 

 veins of porphyry : and since it would be inconsistent and unphi- 

 losophical to assign the production of phenomena of the same cha- 

 racter to different causes, the probable origin of all veins, either by 

 injection or sublimation from below, receives from these facts new 

 and independent support. 



The spirited publications of Mr. Scrope, especially his plates 



* Phil. Trans.; 1822 and 1823. 



