4 SHALER ON THE CLASSIFICATION OF LAVAS 



The Lavas of Ordinary Volcanos. 



The immediate problems connected with the formation and extrusion of the lavas from 

 volcanic cones are perhaps on the Avhole less perplexing than those we shall have to 

 consider in the other classes of volcanic outbreaks; nevertheless, we find ourselves at the 

 very outset in the midst of a maze of discordant opinions. After carefully weighing 

 the diverse facts and theories which have been accumulated about the problem as 

 far as they are known to me, I am inclined to maintain the following propositions as 

 sufficiently supported to be worthy of presentation. 



1. The formation and ejection of the lavas of ordinary volcanos is due to 

 the invasions of heat into sedimentary deposits ; invasions brought about by 

 the deposition of sediments on the sea-floors, and the consequent vaporization 

 of the Avaters and other volatilizable substances contained in the deeply-buried 

 rocks, — which substances were deposited at the time the buried rocks were laid down. 



This proposition is supported by the following facts, viz. : In the first place, the 

 active volcanos of the earth are limited to the vicinity of areas of sedimentation, 

 and their activity ceases when in the course of geographical change, the seas leave 

 their neighborhood. Several times in the discussion of this problem, the suggestion has 

 been made that the water which contributes the larger part of the gases poured out by 

 volcanos, penetrated from the surfiice through the rocks to the heated deep-lying beds, 

 and being there vaporized was the source of the gases which propel the lavas and 

 other substances from the volcanic vent. Although Daubree has shown, by some 

 very interesting and singularly overlooked experiments, that water will penetrate 

 rocks against a considerable gaseous pressure, this theory seems to me essentially 

 untenable, inasmuch as the penetration of water through the rocks beneath the 

 land must be even more energetic than through the rocks beneath the sea, for 

 the reason that the hydrostatic column is higher beneath the land ; we are therefore 

 driven to regard the water contained in the gases of the volcano as originating in 

 the water that is imprisoned in the rocks by stratification. In the second place 

 all the lavas that have been submitted to a careful microscopic study, show that 

 the temperatures to which they have been subjected are not high enough to melt 

 any other than the easily fusible rocks. This is proven by the numerous fragments 

 of more refractory rocli;s which they frequently contain. This is evidence of a very 

 fair kind, to show that lavas are not derived from great depths, and that they are formed 

 from rocks which had previously been consolidated. 



That the ejection of the lavas from ordinary volcanos is due to the action of the gases, 

 is pretty well proven by the fact that most volcanic eruptions are essentially explosions of 

 gaseous substances. Inasmuch as these gases are diffused in the atmosphere, or precipitated 

 as rain, they leave no distinct record of themselves, but there is no reason to doubt that the 

 volume of gases which have been poured out, by such a volcano as ^tna for instance, must 

 amount to many thousand times the mass of the cone that we find there. At the time of 

 fullest activity, such craters are discharging gas with a greater speed than the gases of 

 gunpowder are discharged from a cannon's mouth at the moment of explosion, for they 

 sent their ejection higher than they could be sent by any artillery. In many volcanos this 



