DALY. — THE NATURE OF VOLC.VXIC ACTION. 59 



with simultaneous extrusion, for solution of foreign rock must take con- 

 siderable time. The observed average size of the feeding channels 

 (dikes) in the great lava-fields of the western United States, of North- 

 western Europe, of India, and elsewhere, corresponds with the furmer 

 conclusion. The Icelandic How of 17S3 and the nature of the individ- 

 ual basaltic flows in every prehistoric lava-field, show or suggest that 

 each extrusion has been rapid. The controlling condition for the lack 

 of assimilation is probably the narrowness of the abyssal injections, at 

 least in the part traversing the sedimentary and acid shells of the earth. 



The effusion of a basaltic flood is usually ascribed to the mere squeez- 

 ing out of the magma from beneath a cracked and sinking earth-crust. 

 Yet some force may also be available from the expansion of the sub- 

 stratum material as it rises to levels of enormously lessened pressure. 

 This expansion is of two kinds ; — that of the lava regarded as bubble- 

 free, and that of the gases separated from it in bubble form. If the 

 expansive energy of the liquid proper is not all expended in driving 

 asunder the walls of the injected body, some of that great force is 

 available for extrusion. As magma nears the surface, the separation 

 of the dissolved gas must still further increase the volume and tend to 

 cause outflow at the surface. The relative importance of these three 

 conditions for extrusion is by no means apparent, though the writer be- 

 lieves that the expansional energy of the injected liquid should have 

 more attention than it has had in general treatises on igneous action. 



The fact that the great bulk of visible igneous rock is intrusive, and 

 the related fact that most of the larger Paleozoic and later injections 

 have not extended to the surface, suggest that the upper part of the 

 earth's cru.st has long been comparatively difiicidt of penetration by 

 abyssal magma. It seems fair to hold that a leading cause of this rel- 

 ative impenetrability is the state of compression in the outermost shell 

 of the crust. This compressive stress is relieved b}' an orogenic i)ar- 

 oxysm. After each paroxysm, tensions in the same shell are produced 

 by the cooling of the rocks which had been heated by shearing. For 

 a double reason, therefore, fissure eruptions should be more numerous 

 and of greater volume in periods subse(iuent to strong mountain-build- 

 ing. This expectation is fairly matched by the facts of geological his- 

 tory, as shown in the accompanying table. 



Locality. Date of Fiiwure Fruption. PreccdinK OroRcnic roriod. 



Ijake Superior Keweenawan. Clo.se of the Animikie. 



Distri«;t. 



Rocky Mts. at Middle Cambrian (?) Eariy Middle Cambrian (?) 



4'.»th Parallel. 



