3i6 REPORT— 1894. 



tive outlets from which lava streams have issued far away from the sum- 

 mit, and have thus diminished the general slope of the volcano. Those 

 mountains are usually considered to have an average slope of 10'^. The 

 Hawaiian lavas are, as is well known, exceptionally fluid, and we could 

 hardly expect cones of greater slope than 10^. At Etna the lavas have 

 always been more viscous from their lower temperature and the compound 

 or false viscosity given to them by the large number of porphyritic crystals 

 already existing in the magma at the time of emission, just as earth mixed 

 with water may produce a viscous mud. These new lavas of Vesuvius, as 

 is the case with all those that issue high up on the volcano and in small 

 quantities, were very viscous owing to their low temperature and advanced 

 •crystallisation, so that soon after the material poured out it was prevented 

 from flowing by slight further cooling. We may take therefore this aver- 

 age slope of 14° as the best and most correct estimate for a lava of this 

 nature. 



This recent outflow exhibits most of the varieties of surface to be met 

 v/ith in the type of lava above mentioned, such as corded shapes of diflerent 

 kinds, irregular globular surfaces, sheets, and plates either in position or 

 reared on end, and tunnels of every variety, frequently with continua- 

 tions as walled canals, of which a good example is seen in the photograph 

 ■exhibited. A magnificent lava hump is to be seen in another photograpli, 

 and was formed right under the escarpment of Somma. The origin of 

 these humps is still obscure. They are common on most large flows of 

 corded lava of Vesuvius, but unfortunately 1 have never been present at 

 their formation, nor do I know of anyone who has. 



The points of issue of the lava occurred at various spots along a line cor- 

 responding with the strike of the radial dyke to which it owes its origin, so 

 that the new lava has as a summit an irregular ridge running nearly north 

 and south. Of course the actual highest point is nearly always that where 

 the last lava issued. Generally more than one spot along this line gave out 

 lava at the same time. The fluid rock flowed sometimes on one side, 

 sometimes on the other, so that the general public at Naples were only 

 irom time to time treated to a glimpse of Nature's fireworks, and when the 

 lava flowed in the opposite direction it was often announced that it had 

 altogether stopped. 



During the last year several new conical spiracles were formed, but 

 none of them comparable in perfection of form to those described in tlie 

 last two reports, or exhibiting equally interesting features. 



No very interesting minerals were produced as sublimates. In fact, 

 •OTily two species are worthy of mention. On one occasion a small quantity of 

 tenorite was formed in one of the spiracles. Soon after the lava had entirely 

 stopped flowing in February, sublimates of potash-bearing halite were 

 very abundant around about the vents, in beautiful fern-like skeletons, in 

 which a number of feathery branches radiated at right angles from a stem 

 representing usually about three edges of a cube, and were themselves so 

 many edges of smaller cubes. Sometimes this halite was grey, from minute 

 lirematite crystals being deposited with the salt, which likewise was in 

 :some cases greenish from copper impurities. Most, however, was of a 

 teautiful snow white. One small cave in particular, about the size of a 

 man's body, was clothed with the most glistening white lining, and from 

 the roof and walls showers of crystals fell from time to time. These were 

 not visibly red-hot in bright diflfused daylight, but looking towards the 

 shaded inner extremity of the cavity a bright red incandescence was visible. 



