522 THE POPULAR SCIENCE MONTHLY. 



remark that the crust abounds most in the oxides of those metals 

 which have the strongest affinity for oxygen, as the alkalies and alka- 

 line earths ; while in the peridotic and lower zones the proportion of 

 these elements is much less, and that of the earths and metals is much 

 greater. The minerals composing the superficial crystalline rocks, as 

 well as water, are generally absent from the meteorites. This is espe- 

 cially noticed in respect to the mineral quartz or silica, so common at 

 the surface. According to these views, the granites must once have 

 been in a melted condition, and the excess of silica present in them 

 have assumed the amorphous form. Many geologists have supposed 

 the silica ought to have crystallized first, if the rock cooled from fu- 

 sion. It may be that our ideas of the intense heat have been ex- 

 aggerated ; yet the Labrador granites of New Hampshire have recently 

 been shown by us to be situated in sheets over a plain, precisely like 

 the erupted lava of the present day. 



We have dwelt upon the present concentric structure of the earth, 

 because it was probably the same with that existing in the igneous 

 period, at that time fused, but now largely solid. The order of the 

 alternations has always been the same. It corresponds also with that 

 observed in furnaces, where the metal sinks to the bottom, and is over- 

 laid by one or more successive layers of slag. 



This complex sphere, when molten, with its fiery billows and igne- 

 ous currents, being situated in a fearfully cold region, could not fail to 

 radiate heat ; and, like other melted bodies, become covered with a 

 congealed crust. A pot of melted iron taken out of the fire loses heat, 

 and a crust speedily forms over it, shrinking as it cools ; and, if the 

 exterior be broken, the red liquid may be poured out. The same thing 

 may be seen on the dumping-heaps connected with melting-works. 

 Masses of slag, with their entire surface congealed, are placed upon 

 the car and wheeled to the end of the pile ; but, when thrown down 

 the slope, they are fractured, and the liquid interior flows out like 

 water. When a stream of lava flows down a slope, the surface and 

 sides of the molten river are soon covered by a thick crust, the result 

 of cooling. This will become so firm that men may walk upon it, as 

 upon ice over lakes in the winter. During one of the eruptions from 

 Vesuvius, when lava covered the town of Resina — the old Hercula- 

 neum — some of the inhabitants, driven to the tops of the houses, 

 escaped by walking over the stiffened crust, before the flow had ceased. 

 Whenever the lateral walls of the stream are broken, the lava will flow 

 out and change its course. In this way, a current threatening to en- 

 gulf a village may be averted and directed elsewhere. This is a prac- 

 tical matter, and has been turned to account in Sicily, in warding off 

 from Catania the threatened calamity rolling down the slopes of Etna. 



Our entire experience, therefore, of analogous phenomena, leads us 

 to believe that a crust will be formed, and that the several zones will 

 cool in natural order in later periods. Not till the last melted layer 



