8 SHALER ON THE CLASSIFICATION OF LAVAS 



and in Transylvania, and less certainly elsewhere. It seems to be pretty clearly made out 

 that the escape of these lavas to the surface has been through a fissure of considerable 

 extent, and not through the cones of ordinary volcanos. It is also probable that there has 

 been no great uprush of gases at work in propelling these lavas. It is easily seen that 

 these eruptions present a problem of no small difficulty to those who may hold the opinions 

 concerning the origin of lavas which I have embodied in the preceding propositions. If 

 lavas are produced in the essentially solid earth and extruded by the action of heat oper- 

 ating through the motion of imprisoned gases, how is it that we have these very exten- 

 sive outflows of lava taking place without the aid of any great amount of gas? It seems 

 to me that it is important that these difficulties should be met, and shall therefore 

 present certain considerations which seem to me to offer a fair explanation of the means 

 whereby such lavas come to the surface. 



1. The massive lavas are only found in regions where volcanic activity and mountain- 

 building forces exist in the same field. 



As the geographical distribution of volcanos has been a matter of the utmost conse- 

 quence to the theories of their origin, we may fairly begin our study of this par- 

 ticular species of volcanic activity by considering its distribution. It seems to me to 

 be a fact of the utmost importance to us, that the massive lavas are entirely wanting 

 in many regions characterized by intense volcanic activity, yet remote from mountains 

 which are the product of recent uplifts, and that such outflows are equally wanting in 

 many great chains, where the mountain-building forces have been exercised with the 

 greatest energy. 



In the whole of the Alpine district, where the most intense disturbance of the crust 

 can be seen, or the Appalachian axis, where throughout the whole of the recorded 

 geological time the mountain-building forces have been in operation, these massive 

 lavas are entirely wanting. It seems evident, therefore, that molten rock is not 

 of constant occurrence on the deeper sections beneath the ground whereon our high 

 mountains rest ; that its formation and extrusion probably depend upon the combination 

 of mountain-building forces, and the ordinary volcanic conditions in the same territory. 

 The only way in which these two classes of forces can combine to produce such lavas 

 may be represented in the following proposition. 



2. The lavas thrown out in massive eruptions have been slowly accumulated beneath 

 the surface of the earth near the foci of ordinary volcanos, and owe their ejec- 

 tion to the strong lateral pressure brought to bear upon them by the compressive 

 movements caused by the mountain-building forces. 



There can be no doubt that at a certain depth beneath volcanic cones there is a 

 great mass of lava more or less perfectly fluid, which may remain in this condition 

 for geological periods, after the outbreaks of gases have ceased to take place. This 

 lava parts with its heat with such slowness, that if the mass and the depth at which 

 it lies are considerable, there is hardly any limit to the length of time during which 

 it may remain molten. Now Avhen the mountain-building forces bring very great 

 pressure to bear upon such lavas, they will tend to yield to the strain more easily than 

 the solid rocks, and will be forced up through the old rocks, or through the fissures 



