Reports and Proceedings — Geological Society of London. 333 



them. They represented that line of active and extinct volcanos which 

 stretches from the Gulf of Mexico on the east to the Pacific Ocean 

 on the west, including Orizaba, Popocatepetl, Jurullo, and Colima. 

 A short extract from the publications of the Mexican Geological 

 Survey, translated by Miss Breton, was read respecting the forma- 

 tion of Popocatepetl ('smoking mountain'), showing how the 

 successive periods of volcanic energy may be marked (a) by a period 

 of lava-flow, (6) by one of breccia, and finally (c) by one of ashes. 

 The angles of slope of the volcanos were drawn to scale. 



Professor H. J. Johnston-Lavis exhibited upward of forty lantern- 

 slide views to illustrate the late eruption of Vesuvius and its efi'ects. 

 Nearly all these were taken by the exhibitor, who explained the 

 different phenomena portrayed. He considered this eruption to 

 resemble mostly that of 1822, although the present crater was 

 larger, attaining 1500 feet both north-by-south and east-by-west ; 

 it was probably 500 to 600 feet deep at least. The remarkable 

 character about this eruption was the large amount of fragmentary 

 material ejected, especially in a north-easterly direction, crushing in 

 the roofs of the buildings in the towns of Ottajano, San Giuseppe, 

 and Terzigno. At the first-named locality the depth attained wa* 

 about 0-75 metre, made up as follows : — 

 m. 

 0-04 Grey dust. 



0'49 Eeddish lapilli, chiefly ' supplementary ejecta.' 

 0-20 Black vesicular scoria, chiefly the ' essential ejecta.' 



The material which fell at the Observatory and Naples had much 

 the same arrangement, but was of course less, and practically only 

 sand and dust. Near the base of the cone the ejecta attain to blocks 

 several tons in weight ; and it may be estimated that, at the north- 

 eastern toe of the great cone, in some places the debris must be 

 60 feet thick. It is to be seen as much as 30 feet in thickness in 

 the new ravines that have been formed 



After careful study, the speaker had come to the conclusion that 

 the remarkably uniform and deep scoring of the cone by very regular 

 'barrancos' was due to the sliding and avalanche-like efi'ect of the 

 rapidly accumulating fragmentary material on the steep slopes, and 

 not due to water action. 



The mountain seems to have opened at four, if not five, difi'erent 

 places on the south-western, southern, and south-eastern sides, giving 

 rise to at least three important streams of lava. Another rift, to the 

 north-north-east of the cone, emitted lava that forms an apron on 

 that side of the mountain, and must of course have been formed 

 early in the eruption, that is, before the 7th to 8th of April. 



The ejected blocks are chiefly old lavas and scoria, partly re- 

 cooked and metamorphosed, with their cavities filled by tachylytic- 

 juice from the fluid magma of the neighbouring chimney. Tlie 

 cavities are also often lined by sublimations of augite, hornblenclt% 

 leucite, microsomite, hasmatite, halite, and a well-crystallized yellow 

 deliquescent mineral, which proves to be a new chloride of manga- 

 nese and potash, for which a new mineral name is proposed. A few 



