NATURAL SECTION OF THE CRUST. 



mountains were dry land, while the greater part of the continent 

 of Europe was covered by the ancient ocean, and long before the 

 Alps were reared above its surface. 



In a subsequent tract this subject of the relative ages of the 

 mountain system, will be more fully developed. 



87. From what has been here explained, and from inspection 

 of figs. 26 to 29, the manner in which geologists have been 

 enabled to analyse the crust of the earth to depths far exceeding- 

 any which could be reached by direct excavation, mining, or 

 boring, will be easily understood. Nature herself, by these pro- 

 digious disruptions, has exposed to view the structure of the 

 sedimentary strata in all cases where no subsequent deposition 

 has taken place over them. By such disruptions it often happens 

 that the strata are inclined over a large extent of country, the 

 surface of which, intersecting their planes at a very oblique 

 angle, is necessarily formed by the section of the strata in the 

 order of their superposition, the breadth of the severa 1 sections 

 being so much the greater, the more oblique the angle formed by 

 the horizontal plane with the planes of the strata. 



In fig. 30 the strata are represented inclined very slightly from 



Fig. 30. 



the perpendicular, and consequently the breadth of each stratum 

 at the surface is very little greater than its actual thickness ; but 

 it will be easily understood that if the obliquity of the strata to a 

 level plain were greater, a very thin stratum would present at the 

 surface a considerable breadth. Supposing then the complete 

 series of strata in any tract of country to be inclined at a very 

 oblique angle to the level plain, it will be easily understood that 

 in travelling in a direction at right angles to the lines of intersec- 

 tion of the strata with the surface, the succession of strata may 

 be examined, and a much greater extent of their thickness sub- 

 mitted to observation, than could be accomplished by any artificial 

 sections of horizontal strata. 



88. Among the indications afforded by discordant stratifications 

 of past convulsions to which the land has been subject, are some 



75 



