THE CRUST OF THE EARTH. 



deposition of the lowest of the superincumbent horizontal strata. 

 The date of the disruption is therefore fixed geologically at some 

 period between those of the deposition of the two strata just 

 mentioned. Now whenever it so happens that the lowest, and, 

 therefore, the first deposited of the horizontal strata, stands the 

 next in order above th$ highest of the inclined strata in the 

 geological scale, the date of the disruption is geologically fixed, 

 being at the epoch between the deposition of two strata which 

 follow each other in immediate succession. 



But if, as often happens, the lowest of the horizontal strata 

 hold a place in the geological scale separated from the highest of 

 the inclined strata by several intermediate layers, which are 

 locally absent, then the date of the catastrophe becomes more 

 uncertain, inasmuch as it may have taken place at any epoch 

 between those of the depositions of the highest of the inclined and 

 the lowest of the horizontal strata. 



86. Nothing is more beautiful or conclusive than the reasoning 

 by which the geological dates of mountain-ranges have been 

 determined by these means. One of the most interesting con- 

 sequences resulting from the observation of such discordant 

 stratifications as are here described, is the means they afford 

 geologists of determining the relative ages of different ranges of 

 mountains. Thus, for example, it is easily demonstrated that the 

 mountains of Cumberland, Lammermuir, and the Grampians were 

 raised above the surface of the ocean long before the Alps had 

 burst the crust of the earth, and before even the continent of 

 Europe was dry land. An examination of the slopes cf these 

 British ranges shows that the strata dislocated and inclined are 

 those of the old slate and limestone, while the level strata super- 

 posed upon them in the adjacent plain are the more recent ones of 

 the red sandstone. It follows that these ranges were raised above 

 the waters posterior to the epoch of the deposition of the old slate 

 and limestone, but antecedently to that of the red sandstone ; and 

 since the red sandstone has been deposited horizontally along their 

 base, it follows that the land surrounding them was covered with 

 water subsequently to their elevation. 



An examination of the Alps gives a very different result. On 

 the flanks of these mountains the tertiary strata are found 

 inclined, sloping downwards, until they become level upon the 

 general surface of Europe. It follows, therefore, that the date of 

 the disruption to which the Alps owe their elevation was poste- 

 rior to the deposition of the tertiary strata upon the European 

 continent, while the elevation of the British ranges above men- 

 tioned was anterior to the deposition of the red sandstone ; from 

 which it follows that the Grampian, Lammermuir, and Cumbrian 

 74 



