528 



NA TURE 



[September 20, 1906 



The Pelean type goes a stage further. In it a large 

 proportion of the eruptive magma is blown to fine powder 

 by the expansion of the gases it contains, and thus a 

 mixture is produced of volcanic gases and incandescent dust 

 in which each particle is surrounded and cushioned by an 

 extremely thin layer of gas at a very high temperature, and 

 therefore excessively mobile. The whole mass is therefore 

 endowed with the mobility of a liquid, and under the 

 influence of gravity rolls down slopes on which ordinary 

 solids would lodge. This explanation of the hot blast 

 which destroyed St. Pierre was first advanced by Dr. 

 Flett and the lecturer in 1902, after witnessing an eruption 

 of Mont Pel^e, and it has since been generally adopted.' 



In that year the Wallibu Valley at the foot of the 

 Soufriere, in St. Vincent, was filled by such an incan- 

 descent avalanche to a depth of 80 feet, and the Rabaka 

 Valley to a still greater depth. The torrential tropical 

 rains, descending these valleys after the eruption, came in 

 contact with this hot ash and caused various secondary 

 phenomena, such as steam explosions, falls of ash, and 

 gushes of boiling mud, which the lecturer compared with 

 analogous, though somewhat different, phenomena during 

 the late eruption of Vesuvius. 



This eruption, as is usual with those of Vesuvius, pre- 

 sented features both of the explosive and efflusive types, 

 i.e. explosions took place from the central crater, while 

 a great fissure traversed the cone from north to south, and 

 lava was discharged both to the north into the Atrio del 

 Cavallo and also from, chiefly, three or four more hocce, 

 or mouths, along the fissure to the south, which descended 

 in the direction of Bosco Reale, Bosco Trecase, and Torre 

 .\nnunziata. 



The chief interest of the eruption of Vesuvius, however, 

 undoubtedly centred round the explosions, the ejecta, and 

 the secondary phenomena in connection with them. The 

 volcano was in unusual activity in April and May, 1905, and 

 had never been absolutely quiet since that time. From 

 .'\pril 4 to April 8, 1906, and to a less degree later, a series 

 of explosions took place which enlarged the great central 

 crater to an average diameter of more than a quarter of 

 a mile (as measured by Prof. Loczy), removed the highest 

 central part of the cone, and thus reduced the height of 

 the volcano by about 350 feet, as measured barometrically 

 by the lecturer's party. The resulting debris was dis- 

 tributed in part over the flanks of the cone, while a larger 

 amount of smaller material was carried to the other side 

 i:f Somma as far as Ottajano and San Giuseppe. In these 

 villages it attained a depth of 3 feet to 4 feet, and broke 

 down the roofs of many houses, and more than one 

 church. In that of Giuseppe, about 250 persons, who had 

 taken refuge, were buried by the debris and met their 

 death. The crater as seen by the lecturer's party on two 

 ascents was oval or heart-shaped, the longer diameter 

 being north and south, the walls sloping somewhat at the 

 top, while lower down they were precipitous, especially 

 towards the south side. At the north side the slopes were 

 somewhat more gentle and the crater wider, while the 

 lip was much lower, and broken down into a sort of plain 

 some yards wide. This section of the crater corresponds 

 exactly with a diagram in a report by the Academy of 

 Sciences of Naples on the eruption of 1737. This erup- 

 tion seems to have been very similar to that of the present 

 year, and, as in this case, several persons lost their lives 

 at Ottajano. There can be little doubt that the projection 

 of fragmentary material in the direction of Ottajano was 

 in both cases principally due to the shape of the crater 

 when thus, so to speak, re-excavated. A contributing effect 

 was the south-west wind, which blew so strongly as to 

 carry some of the finer material as far as Nola. The 

 wind's effect was to be clearly seen in Naples, where 

 several inches of dust were deposited. 



The larger ejected blocks fell chiefly during the earlier 

 part of the eruption on the slopes of the cone and round 

 its foot, where they were mingled, and to a large extent 

 covered up, with much ashes and scorise. Here were to 

 be seen the most interesting phenomena of the eruption, 

 viz. the great ash slides. The cone was previously almost 

 smooth and very regular in outline. It consisted of lava 



1 See Anderson and Flete, Phil. Trans., series A, vol. c ., p. 353 ct scq. 

 (1903) ; also Anderson, Gnograjikical J oumal, March, 1903. 



NO. 1925, VOL. 74] 



streams and dark coloured ash; in April, 1906, it was 

 thickly covered with whitish ash. This, when it attained 

 a certain thickness, peeled ollf in veritable avalanches and 

 slid down the mountain. The tracks present a radial 

 appearance, and did so before any rain had fallen. It 

 seems likely that the well-known umbrella-like markings 

 on volcanic cones of tuff (consolidated ash), which have 

 usually been attributed solely to erosion by rain, may in 

 some cases, at any rate, be due to this cause. The 

 avalanches were of sufficient power to carry away the Cook 

 Railway. In one part below the funicular station the 

 rails were bent like wire, and remained for a hundred 

 yards or more along the sides of the avalanche track at 

 right angles to their former position. They were kept 

 together by their fish-plates, but had been entirely stripped 

 of the sleepers. No stratification or particular structure 

 in the materials brought down was noticed. .'V look out 

 was kept for Lava del Fango (mud lava), which has often 

 caused much damage after Vesuvian eruptions, but there 

 had been no rain to form it before the lecturer's arrival, 

 and comparatively little fell during his visit, so that only 

 very small flows were seen. Prof. Lacroix, however, was 

 fortunate enough to observe a large stream of mud above 

 Ottajano, and he remarked that the resulting breccia was 

 a little harder than the result of the dry avalanches, but 

 presented no particular stratification or other structure by 

 which it could be distinguished from the products of a 

 dry avalanche of the same materials. Consequently no 

 light is thereby thrown on the question whether many 

 tuffs, such, for instance, as those which entomb Pompeii, 

 were deposited, dry or wet. 



The lava of this eruption also deserves mention. Flows 

 occurred from the north and south ends of the fissurr- 

 through the cone above mentioned. That to the north 

 flowed into the Atrio del Cavallo in the early part of the 

 eruption. It was soon covered up with fragmentary ejecta, 

 and at the time of the visit only a few fumaroles remained 

 to mark the course of the fissure. On the south side of 

 the cone three or four hocce, or mouths, opened along the 

 fissure, the streams coalesced, and the lava flowed thus 

 for more than half a mile. It then divided into branches, 

 which to a large extent destroyed the villages of Bosco 

 Trecase and Bosco Reale, and nearly reached Torre 

 Annunziata. It crossed and filled up a cutting on the 

 Circumvesuvian Railway. 



The discourse concluded with a number of photographs 

 of explosions from the crater taken by the lecturer during 

 his .stay of five nights at Cook's Eremo Hotel, near the 

 observatorv. 



THE .ASCENT OF RUWENZORI. 



■jWrR. DOUGLAS W. FRESHFIELD gives in the Times 

 of September 13 some authentic details of the success 

 of the Duke of the .'\bruzzi's expedition to Ruwenzori, 

 from a letter received by him from Signor Vittorio Sella, 

 who accompanied the expedition. Signor Sella wrote to 

 Mr. Freshfield under date July 22, from Fort Portal : — 



" His Royal Highness, accompanied by two Courmayeur 

 guides, climbed all the five highest snowy peaks of 

 Ruwenzori and took from them observations with a 

 mercurial barometer besides a great many bearings with 

 a prismatic compass. Captain Cagni carefully measured a 

 base-line near Bujongolo in order to ascertain the exact 

 distance, between the highest peak of Kiyanja (which he 

 climbed) and the rockshelter Kichuchu. His Royal High- 

 ness will therefore be able to publish a really good and 

 complete sketch map of the snowy portion of the chain. 

 Following in his Royal Highness's footsteps I ascended 

 several high peaks and took many photographs and pano- 

 ramas. I also secured many pictures in tile forests and 

 valleys of Mubulvu and Bugiogo (the largest tributary of 

 the Mubuku), and some telephotographs of the chain from 

 near Butiti. The w^eather, however, was very trying to our 

 patience. From June 12 to July 7 we had not a single 

 really fine day. 



" His Royal Highness from his barometric observations 

 will soon be able to calculate and give the correct heights 

 of the crowning peaks of Ruwenzori, which are several 



