rE) Introduction to Geology. 
of the geological county maps and sections of Mr. Win. 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 fig. 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 180.) We have also seen (figs. 108, 109. 117.) 
that certain isolated portions form outliers, capping the sum- 
