1880.] on Land and Sra in relation to Geohujical Time. 275 



little change in the relative levels of its difterent parts. These alter- 

 nations of depression and elevation have all been apparently of very 

 moderate vertical amount. Over the vast area of Russia, we find, as a 

 rule, that the sediments which have been successively deposited upon 

 it exhibit a most regular stratification, and have undergone little or 

 no metamorphic change ; Silurian clay-slates being represented by 

 hardened clay; and Carboniferous limestone showing itself as an 

 aggregate of compacted (foraminiforal) Fusulince. 



On the other hand, a force acting horizontally against the margin 

 of a previously level area, will throw it into plications, of which the 

 elevated portions will form mountain-ranges. The strata forming 

 these ranges, which show by their contorted condition the enormous 

 lateral thrust to which they have been subjected, always exhibit 

 more or less of metamorphic change ; and this metamorphism is now 

 generally regarded as the effect of heat, acting in conjunction with 

 moisture, and usually under pressure.* The source of this heat 

 is to be found in the very mechanical energy which effects the 

 plication ; resistance to which, as in ordinary friction and compression, 

 will cause it to take that converted form. This plicating process acts 

 along definite lines and bands, the width of which is usually small in 

 proportion to the vast area of the wide continental platforms ; and 

 thus it happens that notwithstanding the enormous height to which 

 the most elevated peaks may be lifted (Mount Everest 29,000 feet), 

 little is added by Mountain-making to the average level of any great 

 continent. But, again, the operation of this lateral thrust is now 

 generally recognized, not merely in the elevation of mountain-ranges, 

 but also in Volcanic action; the fusion of the compressed rocks 

 being, in fact, only a further stage of metamorphism, and being fairly 

 attributable, like it, to the production of heat by the conversion of 

 mechanical force. 



III. The recent progress of Physical Astronomy, again — mainly 

 through the application of the Sj^ectroscoi^e to the study of the phy- 

 sical and chemical conditions of celestial bodies in various stages of 

 aggregation — seems now to have placed it beyond reasonable doubt 

 that the earth has cooled down from the state of a molten mass ; and 

 the probable eff'ect of the progressive cooling and shrinkage of its 

 interior, upon the conformation of the crust which first solidified around 

 it, have been very carefully worked out by Professor Dana ;f an outline 



♦ Professor Hull, the able Superintendent of the Geological Survey of Ireland, 

 has shown that in the level tract of Carboniferous Limestone which there forms a 

 great central plateau, the organic origin of the limestone is very distinct ; whilst in 

 these upheaved and contorted strata of the same rock which form the elevated 

 borders of that plateau, the organic origin of the limestone is completely obscured 

 by metamorphic change, which has given it a sub-crystalline texture. 



t See the chapters on " Dynamical Geology," in the Second Edition of his 

 'Manual of Geology' (1875): and, for a fuller exposition of his views, his 

 Memoir in the ' American Journal of Science,' June to September, 1873. 



u 2 



