514 Notices of Memoirs — Br. Johnsion-Lavis on Vesuvius. 



away from the summit, 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 viscoiis 

 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 average 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 with in the type of lava above mentioned, such as corded shapes 

 of different kinds, irregular globular surfaces, sheets, and plates 

 either in position or reared on end, and tunnels of every variety, 

 frequently with continuations as walled canals, A magnificent lava 

 hump 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, I 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 corresponding 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 from 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 the last two reports, or exhibiting equally interesting features. 



No very interesting minerals were produced as sublimates. In 

 fact, only 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 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 gray, from minute 

 hematite crystals being deposited with the salt, which likewise was 

 in some cases greenish from copper impurities. Most, however, were 

 of a beautiful snow white. One small cavity in particular, about the 



