GASEOUS VOLCANIC PRODUCTS. 



119 



most recent of these dykes or veins being the one which has passed 

 up through all the inferior beds or tables, to form the upper one. 



Fig. 201. Fig. 202. 



Sheets, or tnbles of Lava, with their corresponding Dykes. 



37. The matter that constitutes dykes is rarely porous, except 

 Sometimes on the sides towards the rock encasing it ; it is fre- 

 quently even of a finer grain than the table or bed in which the 

 dyke terminates ; its mass is sometimes divided into prisms per- 

 pendicular to the sides of the fissure, which were the cooling sur- 

 faces. This matter generally re- 

 sists atmospheric influences, and 

 it frequently happens that the 

 surrounding rock being degraded, 

 carried away by external agents, 

 the dyke remains projecting on 

 the side of the escarpment (fig. 

 203), -or even rising out of iheFig. 203. Dyke brought into view by 

 earth like a wall. destruction of surrounding rocks. 



88. Gaseous volcanic products. Volcanic phenomena are ac- 

 companied by the production of great quantities of various gases, 

 some permanent, others condensable or soluble. These products 

 consist for the most part of watery vapour; but they are found to 

 contain also various acids, and other matters sublimated from the 

 volcano. Most of these gases are fatal when breathed. 



Gases, always at a high temperature and mixed with the vapour of water, 

 act powerfully on the solid surrounding matters; they disaggregate and 

 decompose them in all ways, reduce them to powder, to mud, and form new 

 compounds of every kind. This happens in all solfata'ras, where it is often 

 necessary to be cautioned against falling into masses of muddy matter, 

 which is sometimes very hot. But nothing is comparable in this respect to 

 the volcans of Java; the acid and aqueous vapours which are there in great 

 abundance, destroy the rocks and form a paste of them, which speedily 

 becomes incapable of resisting the explosive action of the interior. These 

 fearful eruptions take place, not of lava as in ordinary volcanoes, but of 

 enormous masses of boiling water, charged with sulphuric acid and thick 

 mud, which destroy everything in their way, and cover the whole country 

 with a sulphurous slime the matter of which is called buah. This happened 

 in 1822, on the eruption of Gallung-Gung, which, with earthquakes and 

 horrible noises, was considerably sunk, truncated at the summit, and entirely 

 overturned. Torrents of hot sulphurous water and mud issued from renta 



37. What is the character of the matter constituting dykes ? By wha* 

 means are dykes sometimes naturally brought into view? 



38. Wh.it are the characters of the gaseous products of volcanoes? How 

 do gases affect surrounding solids ? Do volcanoes ever eject mud ? In 



hat condition is lava when jrases are disengaged from it ? 

 31* 



