MODERN ACTION OF RIVERS. 561 



things. Again receding to a remoter period, he detects in the bones of singular crea- 

 tures, somewhat analogous to, though very different from the modern tapir, named by 

 Cuvier, Palceotlieria and Anoplotheria, undoubted proofs of dry land and lacustrine ac- 

 cumulations ; and thus our science enables us to take an analytical view of bygone 

 periods, each sera being distinctly characterized by the monuments which it has left 

 for our examination. 



Modern action of rivers. — Although on a much smaller scale than in more mountain- 

 ous countries where streams descend rapidly to the plains, there are several good ex- 

 amples in this region of the erosive fluviatile process, showing, 1st. That where the 

 rivers descend from high grounds they have worn their way through pre-existing shingle 

 banks of an earlier period. 2ndly. That though the rivers have not formed the gorges 

 through which they pass, yet they have partially deepened their beds by the attrition 

 of the materials which they have hurried on. 3rdly. That they have formed deltas both 

 along their course and at their mouths. 



If, however, we observe the feeble efforts which all the Silurian rivers now make 

 in their passage through the plains, and see that they there merely re-arrange a few ma- 

 terials distributed by former action of water, and occasionally make slight changes in their 

 beds, we shall have a firm conviction, that their present volumes of water would have 

 been perfectly inadequate to excavate the deep hollows through which they occasionally 

 flow. A good example of ordinary fluviatile action is seen in the wide river plain of the 

 Towy, below the town of Llandovery, where the river has so changed its course amid, 

 the wreck of the former detritus, that it now flows half a mile from Llandegjad Church, 

 though it formerly passed close by the building. On the other hand, the Sowdde, a 

 tributary of the Towy, is an excellent example of a stream still highly active. This 

 river, descending from the lofty fans of Caermarthen, hurls down in extraordinary floods 

 bowlders of Old Red Sandstone and coal grits, 2 to 3 feet in diameter, and distributes 

 them with much red mud over an arid flat near the town of Llangadock 1 , where the 

 Sowdde unites with the Towy. But the Sowdde does not break these bowlders from 

 the parent rock. The deep upland valley through which it flows, extending from the 

 base of the Caermarthen Fans by Gwinfe to Castel Cerrig Cennen, and immediately 

 beneath the most elevated and broken portion of the margin of the coal-field, is covered 

 with considerable quantities of coarse detritus accumulated as before described by an- 

 cient submarine action, and the river Sowdde rushing down through these heaps merely 

 transports the ready-made blocks to the vale at Llangadock 2 , 



1 After each flood the Sowdde leaves also a broad superficial deposit of red mud, and when the river is 

 highly saturated with this detritus, the waters of the Towy into which it falls, are discoloured, even to the 

 neighbourhood of Caermarthen. 



8 Other analogous examples of torrential river action might be cited from the precipitous escarpments of the 

 South Welsh coal-field ; for example, the streams descending into the Vale of the Usk, south of Crickhowel, 

 and that at Pontypool quoted by Dr. Buckland, Geol. Trans., Old Series, vol. v. p. 531. 



