348 GEIKIE 



(a) Eruptive. As the name denotes, eruptive or igneous 

 rocks have been ejected from the heated interior of the 

 earth. In a modern volcano lava ascends the central fun- 

 nel, and issuing from the lip of the crater or from lateral 

 fissures pours down the slopes of the cone in sheets of 

 melted rock. The upper surface of the lava column within 

 the volcano is kept in constant ebullition by the rise of 

 steam through its mass. Every now and then a vast body 

 of steam rushes out with a terrific explosion, scattering the 

 melted lava into impalpable dust, and filling the air with 

 ashes and stones, which descend in showers upon the sur- 

 rounding country. At the surface, therefore, igneous rocks 

 appear, partly as masses of congealed lava, and partly as 

 more or less consolidated sheets of dust and stones. But 

 beneath the surface there must be a downward prolongation 

 of the lava column, which no doubt sends out veins into 

 rents of the subterranean rocks. We can suppose that the 

 general aspect of the lava which consolidates at some depth 

 will differ from that which solidifies above ground. 



As a result of the revolutions which the crust of the 

 earth has undergone, the roots of many ancient volcanoes 

 have been laid bare. We have been, as it were, admitted 

 into the secrets of these subterranean laboratories of nature, 

 and have learned much regarding the mechanism of vol- 

 canic action which we could never have discovered from 

 any modern volcano. Thus, while on the one hand we meet 

 with beds of lava and consolidated volcanic ashes which 

 were undoubtedly erupted at the surface of the ground in 

 ancient periods, and were subsequently buried deep beneath 

 sedimentary accumulations now removed, on the other hand 

 we find masses of igneous rock which certainly never came 

 near the surface, but must have been arrested in their 

 ascent from below while still at a great depth, and have 

 been laid bare to the light after the removal of the pile of 

 rock under which they originally lay. 



By noting these and other characters, geologists have 

 learnt that, besides the regions of still active volcanoes, there 

 are few large areas of the earth's surface where proofs of 

 former volcanic action or of the protrusion of igneous rocks 

 may not be found. The crust of the earth, crumpled and 



