BY THE CONDITIOXS OF THEIR EXTRUSION. 9 



which would form in the movement of the superjacent beds. We see in all our 

 great mountain systems evidences of a certain spewing out of the deeper lyino- beds. 

 The well-recognized fan structure of the Alpine and other massifs, exhibit this. Even in 

 the simpler monoclinal mountains, such as the Malvern Hills, on the line between 

 Worcestershire and Herefordshire, we often have evidences of the forcing ujj of these 

 lowest lying rocks, though they never, so far as is known to me, show the ejected 

 rocks flowing in the fashion of lavas, except _ where there is reason to believe that 

 these regions had formerly been the seat of true volcanic action. 



The Cordilleras of North and South America, the Deccan and the Transylvania 

 district are all regions where this pressure could naturally come upon the reservoirs of lava 

 through the action of the compressive mountain-building forces. On the other hand 

 owing to the fact that volcanos are not not often associated with mountain chains, but are 

 more commonly apart from such axes, we have few regions where mountain-building 

 forces have been brought to bear upon volcanos, so we cannot properly be surprised at the 

 rarity of the occurrence of massive lavas, if we put the interpretation on their origin which 

 is here suggested. It seems to me pretty clear that there is no other power except this 

 compressive action of the mountain-building forces which can be adduced to explain the 

 outflow of lavas when they are not impelled up by the gases of ordinary eruptions. We 

 see that in such eruptions from cones there is great difficulty in forcing the lava out. 

 even by the aid of the most powerful uprush of gas ; now as in the massive eruptions the 

 gaseous element of the eruption was evidently small in quantity, we must suppose that 

 some other form of pressure had been brought to bear upon the lava, and this could not 

 well be any other than that arising from the horizontal compression of the beds that 

 comes about in mountain building. 



It should be remarked that in all ancient volcanic outbreaks we are more apt to have 

 the compact and nearly horizontal lava streams preserved, than the steep and 

 incoherent cones whence they came. No geological features are liable to such rapid 

 eftacement as volcanic cones. The high angle of their slopes, the incoherent nature of the 

 materials that compose them, and the shocks to which they are subjected, lead to 

 their very rapid destruction ; so that the older the volcanic district, the more likely we are 

 to mistake the ruins of a crater lava system for the evidences of a massive outbreak. This 

 consideration should give us caution in the classification of lavas from this point of view. 



Inclosed Lavas. 



This third and last of the groups of lavas includes all dykes of whatever nature that 

 have not been formed in the pipes through which volcano ejections have made their way 

 to the surface, or in other words all lavas deposited in fissures, the upper ends of which 

 were not open to the surface of the ground. In the immediate neighljorhood of volcanic 

 cones these lavas have often been formed in a peculiar and local way as diversions from 

 the main pipes of the volcano. These 1 would exclude from the general class of inclosed 

 lavas, regarding them as mere parts of the true volcanic lavas. Thus limited, this class of 

 lavas remains jjy far the most extensive group of volcanic products. In most of our 

 older crystalline rocks this group of lavas is very numerously represented. We could 

 name many sections having areas of from one hundred to five hundred thousand square 



