Silurian Bocks of Cornwall. 83 



appear to be many objections to tbe opinions of M. Prevost, 

 as they are expressed by him, inasmuch as no allowance is 

 made or admitted for minor disturbances and actual eleva- 

 tions by subterranean forces. His views, however, are well 

 worthy the attention of the geological inquirer. 



The principles explained place the general theory of change 

 of level by contraction upon something better than a hypo- 

 thetical basis, and are believed to explain the actual causes 

 by which the changes have been produced. They coiTespond, 

 moreover, with the view that ruptures, elevations, foldings, 

 and contortions of strata have been produced in the course 

 of contraction. The greater subsidence of the oceanic parts 

 would necessarily occasion that lateral pressure required for 

 the rise and various foldings of the Alleganies and like re- 

 gions. 



A brief Review of the Classification of the Sedimentary Bocks 

 of Cornwall, By Sir Roderick Impey Murchison, G.C. 

 St S., F.R.S., V.P.G.S. & R. Geogr. S., &c., &c. (In a 

 Letter, addressed Sir C. Lemon, Bt., M.P.) Communi- 

 cated for the Journal by the Author. 



My dear Sir Charles, — In compliance with the promise I made 

 when attending your last anniversary meeting at Penzance, I now 



the rise thus of continents, was first presented by Mr Babbage and De la Beche. 

 M. C. Prevost takes the different ground, that all seeming elevations are the 

 result of subsidence. His propositions are as follows (Bulletin de la Soc. Geol. 

 de Prance, xi., 1839 a 1840, p. 186) :— 



" 1. Que le relief de la surface du sol est le resultat de grands affaissements 

 successifs, qui, par contre-coup, et d'une maniere secondaire, ont pu occasioner 

 accidentellenient des elevations absolues, des pressions lat^rales, des ploiements, 

 des plissements, des ruptures, des tasseraents, des failes, etc. ; mais que rien 

 n'autorise a croire que ces divers accidents ont 6te produits par une cause agis- 

 sant sous le sol, c'est-a-dire par une force soulevante ; 



" 2. Que les dislocations du sol sont des eflfets complexes de retrait, de con- 

 traction, de plissement et de chute ;" 



" 3. Que les raatieres ign^es (granites, porphyres, trachytes, basaltes, lavas), 

 loin d'avoir souleve et rompu le sol pour s'echapper, ont seulement profit^ des 

 solutions de continulte qui leur ont etc offertcs par le retrait et les ruptures, 

 pour sortlr, tninter et s'^panchcr au-dehors." 



VOL. XLIII. NO. LXXXV. — JULY 1847. 



