4 INTRODUCTION. 



which cup-shaped hollows are not uncommon. The 

 King's County bowls seem altogether due to eddies, 

 or the piling up of the drift by transverse currents ; 

 while the Wexford cups seem to have been caused by 

 a subsidence in the underlying strata. At first, 

 probably, there were subterranean passages out of the 

 latter; but now these are almost entirely, if not totally, 

 closed, and there is now always more or less water in 

 them, in which aquatic plants grow, which eventually 

 produce a peaty accumulation. These accumulations, 

 if a constant denudation were going on from the sur- 

 rounding slopes, ought to be more or less earthy, or 

 ought to have layers and thin seams of marl or clay 

 in them, marking each summer's growth of the peat. 

 This, however, is not the case, even when the adjoin- 

 ing land is in tillage ; but on the contrary, as where 

 sections can be examined, it is found that the peat 

 tends to creep up over the drift. 



Two or more systems of joints cross one another 

 more usually at acute than at right angles ; and we 

 find, on looking at the map of the world, that the 

 land is more inclined to form triangles than any 

 other kind of figure. The tributaries of rivers or 

 streams on sloping ground generally join them at 

 acute angles, rarely at right angles. It may be 

 asked, If the courses of streams are mainly due to 

 joint-lines and such like, why then are there not 

 networks of valleys, as each joint-line ought to be 



