Eevieics — Ramsai/s Physical Geology. 525 



witli confidence in the latest conclusions at which geologists may 

 arrive, but the answer to this is simply that it cannot be helped. 

 We must not, however, overlook the historical fact that it is possible 

 for a philosopher to alter his opinions for the worse as well as 

 for the better, and even to change his ideas to such an extent as to 

 bring him back to the point whence he set out. 



The late Earl Eosse, a truly great man, commenced his observa- 

 tions in the belief that there are many nebulse incapable of being 

 resolved with the most powerful telescopes ; in other words, that 

 there are nebulse strictly so called in addition to distant nebulous- 

 looking clusters of stars. By and bye, however, he happened to 

 separate a number of the most hazy-looking nebulas into discrete 

 stars, and this led him to conclude that with instruments of suffi- 

 cient space-penetrating power all nebulee might be separated or 

 resolved. After his death, however (if not before), by a new and 

 independent method of observation, in connexion with the adapta- 

 tion of the spectroscope to the telescope, the theory of the stellar 

 nature of all nebulee was dissipated, and astronomers were brought 

 back to the old doctrine of La Place and Herschel I., namely, that 

 there are many diffused nebulosities in which a process of condensa- 

 tion into suns and planets has either not commenced or is more or 

 less advanced. Nor is this somersault to which nebular astronomy 

 has been subjected altogether without a parallel in what may yet be 

 found to have been a chapter in the annals of geology. 



Many years ago the author of the work before us made a survey 

 of South Wales, with a special reference to the effects of Denudation. 

 He noticed the fact that the ravines excavated by the present fresh- 

 water streams exhibit an abrupt commencement on a pre-existing 

 form of ground consisting of plateaux, escarpments, wide valleys, 

 and passes. This partly led him . to conclude that the broader and 

 more open depressions must have been left by the sea, and that the 

 great escarpments of the Black mountain, and the Brecon and Caer- 

 marthen Fans, were formed by the " Atlantic breakers " during one 

 or more submersions of the land. In his Survey Memoir on South 

 Wales the marine theory is ably advocated and beautifully reasoned 

 from the premises assumed, namely that sea-waves, are capable of 

 producing the above effects. It would appear that a more extensive 

 acquaintance with the mode of wave-action on coasts must have 

 partly led Professor Eamsay to see the truth of the great and impor- 

 tant doctrine that littoral wave-action generally tends to form " plains 

 of marine denudation " by shaving off the summits of eminences 

 as they are trying to emerge from the waters, or by wearing back 

 cliff-lines while the land remains at a stationary level, or while it 

 is slowly and uniformly rising — in the latter case causing inclined 

 plains of denudation; and though it follows from this theory that 

 escarpments must have been left where the planing action of the sea 

 left off (and it is simply incredible that all the escarpments so left 

 should since then have vanished, while so many terraced plateaux 

 and table-lands have remained) , the discovery of escarpments in the 

 softer or more vertically varied formations which pertinaciously 



