LYELL — ON CRATERS OF ELEVATION. 



319 



heaval. On some occasions, as proved by the observations of 

 Scacchi, Schmidt, and others, it indicates a collapse, or partial 

 subsidence of the flank of the cone. That an uplifting of the 

 incumbent mass must accompany the injection of liquid matter 

 through fissures which are not perpendicular (Sir Charles notices 

 some such fissures inclined at an angle of 75 degrees to the horizon), 

 no one can deny ; and therefore, v^hile rejecting the theory of a 

 single terminal catastrophe, or any paroxysmal development of the 

 elevating force, we may fairly ascribe no small influence to those dis- 

 turbing operations, by which such innumerable dikes have been 

 formed near the principal centres of eruption. But the great points 

 to be kept in ^^ew are whether the quaquaversal arrangement of the 

 beds in cones like Etna, and the high inclination of the lavas and 

 scoriae are not mainly, and in many cases exclusively, due to eruption ; 

 and whether the upheaving power, granting its intervention, does not 

 play a very subordinate part. Whether, in fact, it is more probable 

 that, following the proposition of M. de Beaumont, a large portion of 

 the lava-beds now dipping at an angle of 28 degTees had an original 

 slope of only 5 or 6 degrees, the remaining 20 degrees being due to 

 upheaval ; or whether the converse may not be more truly assumed, 

 that the 23 degrees may have been the original average inclination, 

 and that the additional 5 or 6 degrees may have been gained by sub- 

 sequent elevatory movements — in other words, that a fifth part alone 

 of the whole dip may be ascribable to elevation. 



The supposed frequent injection of lava in beds conformable to 

 tufaceous strata, and to which Waltershausen attributes much of 

 the upheaval of the mass of Etna, is then subjected to a similar 

 scrutiny and carefully considered. 



" Had the lavas," writes Sir Charles, " which slope away from the 

 ancient centres of Trifoglietto and Mongibello been in great part in- 

 jected between the tuffs, we should have frequently seen them pene- 

 trating through the dikes. But though these last are of so many 

 different ages, and are continually seen to traverse the alternating 

 lavas and tuffs, I could discover no instance of such dikes being in 

 their turn traversed by lavas. It may be asked, how in the escarp- 

 ments of the Yal del Bove can we distinguish a lava which has flowed 

 origmally at the surface from a tabular mass of rock which may have 

 been forced, when in a melted state, into a fissure between two layers 

 of tuff? I reply, that the lava has almost invariably its upper and 

 lower scoriae, and sometimes immediately beneath the latter a layer 

 of burnt tufl", such as I saw in the Balzo di Trifoglietto, at various 

 heights in Monte Zoccalaro, and in the valley of St. Giacomo, where I 

 traced a red tufi" for a great distance, underlying the most powerful 

 of the older lavas. Such red layers are never in direct contact with 

 the central and overlying crystalline stony layer, for there intervenes 

 always a fundamental stratum of fraginentary and scoriaceous matter 

 between the stony bed and the burnt tufi" below. On the other hand, 

 I looked in vain for an instance of some powerful sheet of lava which 

 had one of these brick-red clays ahove as well as below it. Had the 

 VOL. II.- B B 2 



