J4 Introduction to Geology, 



of the geological county maps and sections of Mr. Wm. Smith, 

 published since his great map, are particularly elaborate, and 

 are locally useful in unravelling the intricacies of the more 

 complicated districts. All these of which we have spoken 

 have appeared within the last sixteen years, and attest the 

 rapid progress of this department of art in our own country. 

 In Ireland, it is understood, geological surveys are proceeding 

 simultaneously with the trigonometrical admeasurement of 

 that country, conducted under the direction of the Board of 

 Ordnance. 



An attentive consideration of the circumstances displayed in 

 the secondary deposits seems to lead us to the natural infer- 

 ence that the earth has been subjected to more frequent revo- 

 lutions, since the creation of organic beings than previously to 

 that era. We are chiefly conducted to this conclusion by the 

 immense variety of strata, by the peculiarities of their position, 

 and by the remarkable fact of the extinction of certain genera 

 and species of animals, the succession of new races in more 

 recent beds, and the alternation of strata containing marine, 

 terrestrial, and fresh-water or lacustrine productions, all indi- 

 cating a vast series of geological epochs. To this fact various 

 other concurrent phenomena bear testimony. Among them 

 may be included the oft-recurring instances both of partial and 

 general disruption, in this division of rocks. Sometimes a single 

 formation, or a series of formations, appear to have been sub- 

 jected to the disturbing force, of which the incumbent strata 

 exhibit no traces. This is remarkably exhibited in the Isle 

 of Wight, where the horizontal beds of Headon Hill abut 

 upon the vertical strata, and demonstrate that the convulsion 

 which placed the latter in that position, took place before the 

 formation of the superior beds. (See^^. 17.) 



For further instances, we refer the reader to our First 

 Volume, figures 107. 109. and 114., which were introduced in 

 illustration of unconformable strata. The lias and new 

 red sandstone formations furnish abundant examples of ho- 

 rizontal beds reposing upon highly inclined strata, all indi- 

 cating that, at the time those strata were so displaced, the 

 higher formations had not been deposited. Sometimes the 

 strata appear to have sunk down ; others have been lifted to 

 considerable elevations, and fragments have lodged on the 

 summits of distant mountains. We have seen elsewhere, in the 

 instance of the Weald of Kent, that enormous excavations have 

 been made, that some of the superior formations have been 

 wholly removed from that area, and the interior beds denuded, 

 (figs. 127. to ISO.) We have also seen (figs. 108, 109. 117.) 

 that certain isolated portions form outliers, capping the sum- 



