18 FORMATION OF THE EARTH 



Similar fissures rent the Hercynian and Caledonian folds, and 

 lava beds and streams, remains of ancient molten lava, are 

 found in many places ; but these have solidified so that all 

 the openings from which these fiery torrents issued are now 

 permanently closed. Stratified deposits at first horizontal, 

 which were raised to form the flanks of ancient mountain chains, 

 have either slid over one another or been completely inverted ; 

 the enormous masses thus dislocated have gradually reached 

 considerable distances at times from the place of their origin, 

 carrying along with them debris from the projecting folds 

 encountered. Elsewhere the stratified rock has been broken 

 vertically along the line of fissure whose two edges had changed 

 their relative levels, constituting a. fault. All this gargantuan 

 task was not, of course, accomplished without sudden shocks 

 causing earthquakes. To-day, however, all is consolidated, and 

 in equilibrium, and only in the vicinity of relatively young 

 mountain chains are seismic shocks still felt. 



Theoretically the order of the superposition of the layers 

 horizontally deposited by the waters should indicate their 

 relative age. When these layers have been forced up vertically, 

 folded, reversed, compressed, or carried away by cataclysms, 

 this determination becomes more difficult ; but it is the business 

 of stratigraphers to overcome these difficulties. They are almost 

 always successful, and have developed, in conjunction with 

 stratigraphy, a new science, that of tectonics, the special object 

 of which is the study of the different agencies operating in the 

 laying down of strata in different localities. When these layers 

 have been pushed up vertically or folded, then raised above 

 the water and once more submerged, the waters flowing back 

 over their old domain cover it with horizontal strata oriented 

 in a direction different from that of the tilted strata. This 

 discordance indicates clearly that there were ground move- 

 ments before the new layer was deposited, and if these are 

 also in their turn folded, the discordancy persists, thus showing 

 that the terrain was lifted up on two different occasions. It 

 was by starting from these principles, so very simple in theory 

 but often difficult of application, laid down in former days by 

 Elie de Beaumont, that geologists succeeded in determining 

 the relative age of mountains and arrived at the conclusion 

 that there had been four series of folds whose distribution we 

 have briefly indicated. 



