496 PROCEEDINGS OF THE GEOLOGICAL SOCIETY, [Jan. 5, 



into sulphurets, which would peld sulphuretted hydrogen when 

 decomposed by water and silica or carbonic acid, the latter being 

 the result of the action of silica upon earthy carbonates. We con- 

 ceive the ammonia so often found among the products of volcanos 

 to be evolved from the heated strata, where it exists in part as 

 ready-formed ammonia (which is absorbed from air and water, and 

 pertinaciously retained by argillaceous sediments), and is in part 

 formed by the action of heat upon azotized organic matter pi^esent 

 in these strata, as already maintained by Bischoff *. 'Nor can we 

 hesitate to accept this author's theory of the formation of boracic 

 acid from the decomposition of borates by heat and aqueous vapour f. 



The almost constant presence of remains of infusorial animals in 

 volcanic products, as observed by Ehrenberg, is evidence of the in- 

 terposition of fossiliferous rocks in volcanic phenomena. 



The metamorphism of sediments in situ, their displacement in a 

 pasty condition from igneo-aqueous fusion as plutonic rocks, and 

 their ejection as lavas with attendant gases and vapours are, then, 

 all results of the same cause, and depend upon the differences in 

 the chemical composition of the sediments, temperature, and the 

 depth to which they are buried ; and the unstratificd nucleus of the 

 earth, which is doubtless anhydrous, and, according to the calcula- 

 tions of Messrs. Hopkins and Hennessy, probably solid to a great 

 depth, intei'venes in the phenomena under consideration only as a 

 source of heat. 



§ VIII. The volcanic phenomena of the present day appear, so 

 far as I am aware, to be confined to regions covered by the more 

 recent secondary and tertiary deposits, beneath which we may sup- 

 pose the central heat to be still ascending, a process which has long 

 since ceased in the palaeozoic regions. Both normal metamorphism 

 and volcanic action are generally connected with elevations and 

 foldings of the earth's crust, all of which phenomena we conceive 

 to have a common cause, and to depend upon the accumulation of 

 sediments and the subsidence consequent thereon, as maintained by 

 Mr. James Hall in his theory of mountains. The mechanical de- 

 posits of great thickness are made up of coarse and heavy sediments, 

 and by their alteration yield hard and resisting rocks ; so that sub- 

 sequent elevation and denudation will expose these contorted and 

 altered strata in the form of mountain- chains. Thus the Appala- 

 chians of North America mark the direction and extent of the great 

 accumulation of sediments by the oceanic currents during the whole 

 palaeozoic period ; and, the upper portions having been removed by 

 subsequent denudation, we find the inferior members of the series 

 transformed into crystalline stratified rocks. 



* Lehrbuch der Geologic, vol. ii. pp. 11.5-122. 

 t Ibid. vol. i. p. 669. 



