4 Wynne — Denudation and, its Causes. 



Differences in features of the ground the result of structure. — In. 

 seeking to discover from the forms of the land, the history of 

 their production, we soon become aware that marked differences in 

 them are connected intimately with differences of geological struc- 

 ture, and that the shapes assumed have a stronger relation to such 

 internal circumstances, than to the external agencies by which the 

 forms have been developed. 



The denuding actions in these countries while belonging to either 

 of the kinds, subserial or marine, are complicated by including the 

 operations of ice. It is here manifestly less easy to observe how 

 much of the results are attributable to one kind of denudation or 

 another, than in countries where ice is not believed to have been one 

 of the agencies employed. 



Notwithstanding this glaciation, it will be seen that the general 

 effect produced by denudation upon rocks which have common 

 structural peculiarities, such as horizontal stratification, etc., is nearly 

 identical even in widely-distant localities. To take a few illustra- 

 tions. The nearly flat-bedded and steep-sided hills of millstone grit, 

 near Manchester, the tabular limestone mountains in the north-west 

 of Ireland, the very similarly shaped mountains overlooking Cairo, 

 Suez, and the west coast of the Red Sea, or those forming the scarps 

 of the great Indian trappean table-lands, of which the Western and 

 Malwa Ghats are examples, show a general uniformity of aspect 

 resulting doubtless from their nearly horizontal bedding. Very 

 different from these are the hill-forms taken by contorted rocks 

 which are as widely similar in their irregularity if in nothing else, 

 while the familiar heavy outlines frequently given by denudation to 

 granitic hills is equally well marked. 



Some of these example are taken from a glaciated region, others 

 are exposed to the influences of excessive periodic rains and heat, and 

 others still are situated in what has been called a rainless district ; 

 but all are instances of forms produced by denudation, and their rela- 

 tive similarity is but little affected by circumstances of climate. 



The valleys excavated out of rocks are even more similar in 

 general characteristics than those hills which have a common geo- 

 logical structure ; marked peculiarities being chiefly observed where 

 they may be attributed to peculiar actions, such as that of ice ; 

 although there are some valleys whose denudation presents unusual 

 singularity.^ 



Hence it may be assumed that there is a general tendency in rocks 

 to yield upon the large scale certain similar results under the action 

 of denuding forces. 



Cliffs : Subcerial and Marine. — Marine denudation being known to 

 produce cliffs, these, when they occur inland, have been attribiited to 

 the former action of the sea ; and as it is admitted that most of the 

 land has been frequently beneath the ocean, many inland cliffs must 

 have stood within reach of its denuding influence. But lofty inland 

 cliffs very frequently occur in mountain recesses without such bold 



' See Prof. Ramsay's paper, on Lake Basins. — Quart. Journ. Geol. Soc. vol. xviii. 

 1862. 



