212 Mr. T. M. Reade on the Geological Consequences of a 



the " shell-of-compression " entirely forbids the acceptance 

 of this secondary theory of volcanic energy. 



Taking the greatest estimate of the present depth of the 

 level-of-no-strain, the seat of volcanic energy on Mallet's 

 hypothesis would be limited to a depth of less than 5 miles. 

 When we point out that lava rises up to and flows from 

 orifices of volcanoes at a level in some cases of 1\ miles above 

 the sea, it is obvious to anyone who has the slightest know- 

 ledge of mechanics that the locus of its origin in the earth's 

 crust must be greatly below the highest estimate of the thick- 

 ness of the shell-of-compression. 



Cores of Mountain- Ranges. — The distinguishing charac- 

 teristics of great mountain-chains is, as I have fully pointed 

 out elsewhere, the presence of a central core of gneissic or 

 granitic rock, frequently, as in the case of the Alps, taking 

 the form of a series of ellipsoidal bodies throwing off on the 

 flanks — not seldom with reversed dips — the sedimentary rocks 

 through which they have been protruded. 



This is also true of the great ranges of the Caucasus, the 

 Pyrenees, the Rocky Mountains, the Himalayas, the Andes; 

 and I believe such gneissic cores will be found to distinguish 

 every great range when sufficient observations have been 

 made. The apparent exception of the Appalachian chain is, 

 in my view, no exception at all; and I have already brought 

 forward a considerable body of evidence to prove that the 

 gneissic masses lying to the eastward of the great sedimentary 

 folds is the original core of the range much denuded and 

 deprived of the flanking sedimentary masses formerly existing 

 to the eastward *. 



Whether this explanation be accepted or not, the gneissic 

 and granitic rocks protruded through and often entangling 

 in their folds the great sedimentary deposits which together 

 admittedly constitute most mountain-chains, are a standing 

 monument of the untruth (said in no offensive sense) of the 

 contractional hypotheses of the origin of mountain-ranges. 



These central cores could not have been forced up except 

 accompanied by great lateral pressure, yet now we find, on 

 accurately working out the contractional hypothesis, that the 

 lateral pressure is, taking the highest estimate, zero at a depth 

 of 5 miles, whereas, according to the estimates of accomplished 

 geologists, the sedimentary strata through which the cores 

 were protruded ranged from 5 to 10 miles thick. It is 

 plain to demonstration that the lateral pressure that forced 

 up the mountains could not reside in a shell-of-compression 

 only 5 miles thick having a zero strain on the underside. 

 * Pp. 34 and 35, ' Origin of Mountain-Ranges.' 



