554 THE POPULAR SCIENCE MONTHLY. 



ent land of the globe, though formed in great measure of marine forma- 

 tions, has never lain under the deep sea ; but that its site must always 

 have been near land. Even its thick marine limestones are the depos- 

 its of comparatively shallow water. Whether or not any trace of 

 aboriginal land may now be discoverable, the characters of the most 

 unequivocally marine formations bear emphatic testimony to this prox- 

 imity of a terrestrial surface. The present continental ridges have 

 probably always existed in some form, and as a corollary we may infer 

 that the present deep ocean-basins likewise date from the remotest 

 geological antiquity. 



(b.) Crystalline. While the greater part of the framework of the 

 land has been slowly built up of sedimentary materials, it is abundantly 

 varied by the occurrence of crystalline masses, many of which have 

 been injected in a molten condition into rents underground, or have 

 been poured out in lava-streams at the surface. 



Without entering at all into geological detail, it will be enough for 

 the present purpose to recognize the characters and origin of two great 

 types of crystalline material which have been called respectively the 

 Igneous and Metamorphic. 



1. Igneous. As the .name denotes, igneous rocks have risen from 

 the heated interior of the earth. In a modern volcano, lava ascends 

 the central funnel, 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 exjdosion, 

 scattering the melted lava into impalpable dust, and filling the air with 

 ashes and stones, which descend in showers upon the surrounding coun- 

 try. 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 

 the 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 un- 

 dergone, 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 volcanic 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 be- 



