IGNEOUS INTRUSIONS AND VOLCANOES. 247 



sion advanced on a previous page, that the primary force which 

 causes lava to rise in the conduit of a volcano is not steam pres- 

 sure. 



How molten lava hecomes charged with water can only be 

 conjectured. It is well known that many liquids, especially when 

 highly heated and under heavy pressure, will absorb gases. In a 

 similar way we may conceive that liquid or plastic rock, on coming 

 in contact with water, will absorb the steam produced. 



When molten lava rising in the conduit of a volcano passes 

 through water-charged rocks and nears the surface, pressure is 

 relieved and the occluded steam escapes. This escape is either 

 quiet or explosive, dependent on the nature of the magma in 

 which the steam is dissolved. If the magma is highly fluid, as in 

 the case of many basic lavas when extruded, the steam escapes 

 quietly ; but if the magma is viscous, as is the usual condition of 

 acid lavas when erupted, violent explosions are apt to occur. The 

 quantity of steam absorbed also influences the fusibility of a 

 magma. Apparently the larger the quantity of occluded steam, 

 the more liquid the molten rock becomes. Greater freedom may 

 thus be afi^orded for the passage of a magma in the upper por- 

 tion of the conduit through which it rises than obtains at lower 

 levels. Something of the intermittent character of volcanic 

 eruptions may depend on this cause. Probably, also, the quantity 

 of water present in a magma has an influence on the nature of the 

 minerals formed as it cools. For this reason one would expect 

 differences to appear in the mineralogical composition of rocks 

 formed from magma that have cooled near the surface, and those 

 that failed to reach the water- charged portion of the earth's crust. 



The intimate connection between subterranean injections and 

 volcanoes leads to the suggestion that the domes above intruded 

 magmas may become fractured and give origin to volcanoes which 

 would be supplied by local reservoirs. Something like " craters 

 of elevation " may be thus formed. 



If two or more cisterns of molten rock should be formed in 

 the earth's crust near each other, or at different levels near the 

 same radius of the earth, and fractures formed above them which 

 would admit of the escape of their material to the surface, the 

 striking phenomena of two adjacent volcanoes erupting inde- 

 pendently of each other might result. Such an occurrence is ren- 

 dered more probable by the fact that reservoirs beneath sub- 

 tuberant mountains are supplied through fissures from below, 

 which might become closed, thus isolating bodies of injected ma- 

 terial in the earth's crust. Even if the feeding fissures were not 

 closed, the large cisterns of fused rock to which they lead might 

 discharge some of their material without immediately atlecting 

 the plastic central mass of the earth, which in these sugges- 



