On the Culm-bearitig Beds of Devon and Cornwall. 293 



the copper was clear and bright, and original delicate file 

 marks could be distinguished on it. 



This phaenonienon differs materially from that of inactive 

 iron ; as the latter metal being made positive electrode, evolves 

 oxygen, i. e. acts as an inoxidable metal, and does not arrest 

 the current. 



Admitting the explanation of this latter anomaly, first given 

 by Dr. Faraday (Phil. Mag., 1837), that the iron is by the 

 agency of the electric current closely coated with a protecting 

 film, it would seem that the difference between the case of iron 

 and copper is, that in the former the protecting substance is a 

 conductor, in the latter a non-conductor. The momentary 

 deflexion of the galvanometer is, on this supposition, attribu- 

 table to the incipient electrolyzation, which occasions the 

 formation of the protecting coat. 



XLIII. On the Geological Position of the Culm- and Plant- 

 hearing Beds of Devon and Cornwall. By the Rev. D. 

 Williams, F.G.S. 

 To the Editors of the Philosophical Magazine and Journal, 

 Gentlemen, 

 T SHALL feel obliged by your inserting the following abs- 

 *- tract of the communication I lately made at Birmingham, 

 as it will not only serve as a reply to the article of Mr. 

 Weaver, in your July Number, but I trust effectually set at 

 rest the hitherto disputed position of the culm- and plant-bear- 

 ing beds of Devon and Cornwall. The two apparent instances 

 of unconformity noticed by Mr. Weaver, and indirectly and 

 reluctantly admitted to be ambiguous, are restored to perfect 

 order and parallelism by an overwhelming accumulation of, 

 facts elsewhere, if those instances cited were less equivocal. 

 Working by " inference " is a very unsound basis for a broad 

 and bold generalization, and when its results are too confi- 

 dently enunciated by gentlemen of acknowledged reputation, 

 not only tends to retard the progress of scientific truth, but 

 eventually to damage the reputation of the philosopher for 

 accurate and cautious observation when satisfactorily ex- 

 plained away. Thus at Muddlebridge, where Mr. Weaver 

 " inferred " unconformity, the strike of the Trilobite slates 

 from the east of Bickington, carries them full half a mile to 

 the north of Muddlebridge, the interval being occupied by 

 Coddon Hill grits and slates of a composite and neutral cha- 

 racter. Muddlebridge is not only part of a small area of less 

 than half a mile radius, exhibiting great local derangement, 

 but the fact of the Floriferous Beds there dipping N.E., while 

 the Trilobite Slates, a mile and three quarters to the east, 



