494 E. MALLET ON THE MECHANISM OF 



raised at all. These views, however, tend rather to strengthen 

 Lyell's inferences from the contrary dip of the Etnean beds observed 

 by him. 



Lyell, desirous of doing full justice to the views of M. Elie de 

 Beaumont, admits that the intrusion of volcanic dykes may increase 

 the slope of volcanic mountains. It appears to me, however, that 

 the possible increase of slope thus conceived to be producible has 

 been much overestimated, the following conditions being taken 

 into account : — 



1. The united or total thickness of all the dykes in the escarp- 

 ment of Somma, as examined by me, was a little under 150 feet ; 

 and that is the thickness of the intruded matter in the amphitheatre 

 of 2 miles round the curve, which, regarded as circular, has a mean 

 radius from the axis of Vesuvius of about a mile. If we even 

 double this thickness of intrusive matter in order to allow for that 

 of unobserved broken up or obliterated dykes, intermixed with those 

 surveyed, and bear in mind that the north slope of Somma at the 

 level of the Atrio averages about 25°, it can be easily seen that the 

 increase of inclination, taking the entire original slope of Somma 

 from the summit to the plain at about three miles, would prove 

 very insignificant and might be estimated as still less in reference 

 to the much larger features ef Etna. 



2. Although intrusive dykes may occur at all altitudes, they pre- 

 ponderate in numbers and in thickness about a zone extending 

 hypsometrically to a height very far short of that of the entire 

 mountain, few or none probably reaching any considerable depth 

 below the base, where the horizontal resistance to the Assuring pres- 

 sure is enormous, and but seldom reaching a height greater than 

 about two thirds the entire height of the mountain, because the 

 filling of the crater with liquid lava high above this level is pro- 

 bably not very common. Hence the effect of such intruded dykes 

 must rather be to produce gibbosity or some local increase of slope 

 about the middle zone of volcanic mountains than any general in- 

 crease in the inclination of the sides. 



3. As these dykes are not produced at one time or radially all 

 round the circumference, but singly or a few at a time, and spora- 

 dically distributed where the weakest parts at different times permit, 

 so it seems difficult to assume any noticeable increase of general 

 slope or of inclination of the internal beds as arising from this 

 cause. 



The length of this paper forbids my entering at all upon the 

 mechanism by which so many of the parasitic dust cones existing 

 upon Etna, of which Monte Rosso may be regarded as a type, 

 and found also in the numerous "Puys" of Auvergne, have been 

 opened at one side and become horseshoe-shaped. This has been 

 commonly attributed by authors to their having been burst by the 

 hydrostatic pressure of lava, filling more or less completely these 

 steep cones of almost completely incoherent material; and the 

 streams of lava still observable in some of them at the opened 

 side have been appealed to in proof of this, though the greater 



