THE CRUST OF THE EARTH. 



consequently less and less resisting, so at more and more remote 

 periods, these undulations, and their consequent disruptions and 

 explosions, must have been much more frequent, and attended by 

 catastrophes infinitely more violent. The volcanic eruptions 

 which have taken place within historic times, may be regarded as 

 miniature reproductions oj: the phenomena of which the globe was 

 the theatre at much more remote geological dates. The wear, 

 abrasion, decomposition, and transport of the solid materials of 

 the earth's crust by the action of atmosphere and the waters of 

 the ocean, when continued through periods compared with which 

 that limited by the existence of the human race is but a unit r 

 can easily be imagined to have produced all the effects which 

 are visible on the earth's surface, and to greater or less depths- 

 within its crust. The deposits formed by the detritus of the land 



Fig. 84. 



carried by the currents of rivers to their embouchures, exhibit on 

 a small scale the stratification produced by pre- Adamite seas. In 

 a word, all the geological phenomena discoverable by the sections, 

 natural or artificial, of the earth's crust, admit of clear and satis- 

 factory explanation, by merely imputing to the physical agents- 

 now in operation an energy proportional to the diminished thick- 

 ness of the earth's crust, and effects due to a continuance of action 

 for periods of time, compared with which the common chrono- 

 logical units must be regarded as insignificantly minute. 



Having thus briefly indicated the natural causes to which geo- 

 logical phenomena must be ascribed, we shall resume the subject 

 which we had dropped, and continue our notice of some of these 

 results t which illustrate the past condition of the earth. 



169. There exists in England, in the Isle of Portland, as well 

 as elsewhere, and on various parts of the continent, a stratum 

 called by miners and quarrymen the " dirt-bed." This consists of 

 122 



