CAREER AXD NATURE OF ABSORBED GASES. " 263 



At the same time it must be granted that there are signs of chemical 

 combination going on in lavas. There are minerals like apatite which 

 seem to have been formed gradually at various stages in or all through 

 the development of the rock, from the very earliest time to the latest. 

 Apatite is commonly classed among the earlier formed minerals, and 

 rightly, but as it occurs in the cavities of the lava Capo di Bove or in the 

 interstices of quartz-diabases it might be almost ranked as secondary. 

 Michel-Lev}^^ also has referred to the spinels and magnetite as formed 

 by chemical precipitation. The precipitating agent may well be con- 

 ceived to be occluded oxygen gradually exerting its affinit}^ on basic 

 salts or metals or replacing fluorides. These actions do not seem to be 

 very important, however, in view of the reasons stated in the preceding 

 paragraph, more especially the elevated temperatures at which the 

 tendency to lose gas would come into play. 



It might be supposed, however, .that these gases had alread}^ escaped 

 and now formed the air and water surrounding our globe. Professor 

 Shaler.t for .example, is one of those who have suggested that lunar 

 volcanoes were formed by escaping gases originally absorbed, while he 

 does not look to the same cause for our earthly ones. The only way, 

 however, one can imagine them to have escaped is by a solidification 

 from the center, which gradually drove them out ; i but even if so, the 

 pressure must have been enormous at the center, so that, as shown in 

 Graham's experiments, in which the absorbed gases were given off onl}^ 

 after being subjected some time to the air pump, much gas must have 

 been occluded in the soHd mass. Similarly the waters of the depths of 

 the sea contain under their great pressure more gas than the surface 

 waters ; hence there is no need of imagining with Daubree that the 

 .water or gas has percolated in from the outside. Doubtless it would do 

 so had not the same forces which would draw it in now, kept it in from 

 the beginning. 



Their Nature. — The question naturally arises : What are these gases? 

 We might find suggestions as to their nature from the atmosphere; from 

 the gases given off by the iron and slag of a blast furnace, or from the 

 gases found in meteorites. Some analyses of such gases we group in 

 Table 1. More direct evidence we obtain from the gases and fluids found 

 in the enclosures, § in minerals of the plutonic rocks, and in the gases 

 actuall}^ observed to escape from volcanoes or found in volcanic glasses. || 

 Finally we may obtain some hints from the new minerals of contact- 



* Les Roches Eriiptives, 1889, p. 37. 



t Aspects of tlie^Earth, p. 90. 



tUndeutsch, Neues Jahrbuch, 1893, vol. ii, p. 320. 



^Geikie; Text-book of Geology, 1893, p. 111. 



I Op. cit., p. 193, and Iddings ; Am. Jour. Geology, vol. 1, p. 168. 



