476 E. MALLET ON THE MECHANISM OF 



the escarpment, whose slope they very nearly follow, are the 

 dykes so often referred to by authors. These sometimes run up 

 the entire height of the escarpment, sometimes stop far short of 

 this, sometimes give the observer the idea that they are the edges 

 of straight planes, but often show themselves twisted or curved at 

 the first glance. Many are broken across into separate fragments, 

 which are separated more or less from each other by conglomerate 

 or lapilli. (The general appearance of a large portion of the escarp- 

 ment, as seen from the north flank of the Vesuvian cone, is 

 shown in the panoramic photograph exhibited, for which I am 

 indebted to the photographer who executed it shortly before my 

 survey.) On examining in detail the face of the escarpment, it is 

 observable that the beds of conglomerate or of lapilli present in 

 many places evidences of severe compression in both lateral and 

 vertical directions, and are, over considerable areas, more or less 

 perfectly glued or cemented together by the percolation of water 

 having acted upon the partially soluble silicates of which the dis- 

 integrated material consists, and which have subsequently indurated 

 and formed the cementing material. This mode of forming a com- 

 pact mass, which is not an uncommon volcanic phenomenon, has been 

 observed by myself as taking place in large beds of broken-up slags 

 deposited in the neighbourhood of the Barron Iron Works in Cum- 

 berland. The large quantity of lime employed as flux for smelting 

 the haematite ores operated upon in these vast works produces 

 slags the silicates in which are sufficiently soluble to form a com- 

 pact mass, requiring the pick to break it up at a depth of only 8 or 

 10 feet from the surface, and within a period of about as many 

 years from the time when the slags were in liquid fusion, the water 

 acting upon them being derived from rain only. The bank of the 

 escarpment of Somma thus composed of masses of very variable or 

 of no coherence, is acted upon by rain falling upon its abruptly 

 sloping face ; and in several places deep recesses or barrancos aro 

 cut back into it, some of which are of great size, and have their 

 floors at the level of the Atrio covered more or less with a steep 

 sloping talus of detritus swept down by rain torrents from the lofty 

 bank above. 



On ascending at the western extremity to the crest and following 

 along more or less nearly the edge of the escarpment upon the 

 outer slope of Somma, which falls towards the plain at angles 

 generally of from 25° to 32°, we find it intersected by numberless 

 rain-channels generally tending down the slope ; these in many 

 instances attain the magnitude of large " nullahs," or even of 

 gorges, and, from the loose nature of many of their materials, are 

 not crossed even in dry weather without some danger and difficulty ; 

 indeed the walk along the edge from west to east is extremely la- 

 borious and difficult. It is, however, instructive in several ways, 

 and especially as showing that not very many of the superior or 

 northern terminations of these dykes reached as far as even the 

 existing surface, and were far from ever reaching the surface of the 

 northern slope of Somma, while that mountain still existed as a vol- 



