.Mr Poulett Scrope's Considerations on Volcanos. 353 



face of the ocean/' the author quits the volcanic phenomena, properly so 

 called, to apply the knowledge with which the investigation of these 

 phenomena has furnished him, on the nature and mode of action of sub- 

 terranean caloric, to account for the geological features of the continental 

 formations. And herein appears to consist a main distinction between 

 the geological theory brought forward by Mr Poulett Scrope, and those of 

 Hutton, or other writers on the same subject, who may seem to have fore- 

 stalled him in some of his principal conclusions ; viz. that, while the latter 

 class of theorists directed their efforts to prove that the chief appearances in 

 the constitution of the earth's crust could only, or could most rationally, 

 be explained by the hypothesis of an intense central heat producin" ele- 

 vations, &c, the author we are at present reviewing, directly demon- 

 strates the existence of this central heat, and elevating power, from the 

 phenomena of volcanos and earthquakes ; draws from the same source, 

 conclusive evidence of the laws under which it acts ; and goes on to show, 

 that such a power must, in the nature of things, have given rise to those 

 elevations of continents and mountain ranges, with all the minor pheno- 

 mena of inclined and distorted strata, dikes, veins, faults, &c. which it is 

 one of the chief objects of geological inquiry to account for. 



This chapter commences with the remark, that the arenaceous and sedi- 

 mental strata, which compose the major part of the surface of our con- 

 tinents, are found to assume a great degree of inclination, and more ir- 

 regularities of position, as we approach the chains of mountains, or lines of 

 maximum elevation and disturbance. They, however, almost uni- 

 versally lean against masses of crystalline rocks, which form the geologi- 

 cal axis of every mountain. Of these rocks, some are stratified, or rather 

 have a laminar structure, as gneiss, mica-slate, &c., and show marks of 

 the action of some violent force upon them, in their repeated flexures, 

 cracks, and highly inclined position ; others are unstratified, (granite, 

 syenite, porphyry, serpentine, diallage-rock, and greenstones, &c.,) and 

 usually underlie the others, or cut through them in the manner of im- 

 mense dikes. The latter are supposed by the author to be portions of the 

 subterranean crystalline bed, protruded by inferior expansion, sometimes 

 in a state of partial liquefaction, at others as a solid mass, through a longi- 

 tudinal cleft broken across the superficial strata. The laminated crystal- 

 line rocks, which formed the lower portion of these strata, were forced 

 likewise through the fissure by the tremendous friction of the rising mass, 

 and, during this process, were folded into repeated doublings, like those 

 produced in a bale of cloth or linen, by a powerful pressure, acting nearly 

 in the direction of its layers. In general, the central axis of unstratified 

 crystalline rock, will appear like a vast dike intruded between the repli- 

 cated schists on either side; at others, these protruded strata will still 

 cover the axis like a mantle. Where the temperature of the exposed parts 

 of the crystalline axis was intense, a superficial intumescence may havetaken 

 place, the liquefied matter overspreading the edges of some of the over- 

 lying or protruded strata, and thus giving rise to the appearance of second- 

 ary granites, syenites, porphyries, cvc. Portions of lava will also be in- 

 VOL. IV. NO. II. APRIL 1826. Z 



