10 Wynne — Denudation and its Causes. 



ground, sucli as we have no examples of around the plains which 

 now occupy so much of the centre of Ireland ; but it is, perhaps, 

 hardly fair to speculate as to the entirely marine origin of these 

 plains from the appearances presented where the junction between 

 their old surface and its covering of drift is seen, as we cannot tell 

 how much of these appearances may be due to the action of field-ice 

 or other glacial agencies. However, if we turn to the elevated 

 plateaux of the Deccan in India, they afford even less positive 

 evidence of having been sea-bottoms, in the absence of rolled detritus 

 or wave- worn debris to indicate the approaches to ancient coasts, and 

 yet their character as plains is most strongly marked. 



In many cases we are unable to say what the stratified conditions 

 of the rocks beneath such features may be ; in Tuaxxy others we find 

 a rough parallelism between the stratification and the surface, and 

 in some instances we have underneath them bedded rocks, both flat 

 and undulating, as well as highly inclined. If any rule exists, it 

 may be indicated by the observation that where plains occur, the 

 rocks, if stratified, are often nearly horizontal ; or if inclined and 

 contorted, or of igneous origin, they may have a certain general 

 equality of texture, resulting from a wide extension of one kind of 

 rock, or from a greatly jointed, cleaved or frangible condition, such 

 as would facilitate their destruction by sea-breakers. 



It is no new observation that rain seems to act vertically, its 

 tendency always being to produce steep ground where it is not 

 accumulating materials — thus we are obliged, in the absence of any- 

 thing more likely to produce them, to attribute the formation of 

 plains to the action of the sea. 



Conclusion. — In concluding these observations it need only be said 

 that the consideration of this subject strengthens the conviction that 

 all the forms of the land cannot be fairly attributed to any one kind of 

 denudation with which we are acquauited : — that the similarity of the 

 general results notwithstanding differences ia the causes from which 

 they may have proceeded, and their close connexion with geological 

 structure, involves their origin in some obscurity, which may lead to 

 error, if a prejudice exist in favor of either marine or sub -aerial 

 agency, and that while great changes are effected by the endless 

 action of the sea, the equally continuous atmospheric agencies are 

 sufficiently powerful to produce, in the lapse of time, results so enor- 

 mous, that time also is required for their full appreciation. 



EXPLANATION OF PLATES. 

 Plate I. 



Fig. 1. Sketch of a Trappean hill at "Wassid, near the foot of the Inclines hy which 

 the Great Indian Peninsular Eailway crosses the Western Ghats. 



2. Part of a Sketch from the elevated hill-station of Matheran, between the 



Ghats Proper and Bombay (Trap-rock). 



3. "The Old Man Rock," formed (as well as can be recollected) of purple Grit : 



mouth of Bull's Creek, South side of Dingle Promontory, Ireland. 



4. Sketch from the brow of Benluben Mountain, in Sligo. A. Fissure be- 



ginning to open. J5. Detached pinnacle of rock. 



5. Sketch of part of a long Cliff-terrace on Benluben Mountain, County of 



Sligo. 



