296 Proceedings of the Royal Physical Society. 



gorge, the flood spread over the bank, and sent off some of 

 its top- water in a side track. Even here, then, there existed 

 some of the conditions for the formation of a terrace worked 

 out, after the fashion described in treating of the amphitheatre 

 terrace, by this little diverticulum. 



We now pass on to a consideration of some of the circum- 

 stances of environment that bear upon the formation and 

 preservation of river terraces. 



Valleys, viewed in their relation to Eiver-Terracing. 



The course of a great river is distinguished by Mr Archi- 

 bald Geikie into three regions : First, The Mountain Track, 

 where the young stream, continually swelled by lateral 

 torrents, dashes down mountain sides and through ravines. 

 Second, The Valley Track, where its course is more leisurely, 

 and its rockier parts exist chiefly as gorges between wider 

 and more alluvial reaches of valley. Third, The Plain Track, 

 where the river winds out upon alluvial plains, largely of its 

 own forming, and often deposits more than it erodes. 



In the mountain track streams generally occupy them- 

 selves in deepening their rocky V-shaped channels, and in 

 slowly planing them — if the structure of the strata permit — 

 into an increasingly flat-bottomed U. In the plain track, 

 again, where the river so often winds aimlessly among its 

 own deposits, and planation is limited to the spreading-out 

 of materials in planes, the only chance of terrace-formation 

 lies in upheaval of a coast having sufllcient slope to com- 

 municate activity to the stream. It is in the valley track 

 that terrace- formation can best be studied in active progress. 

 The gorges of hard rock by which most valleys are more or 

 less interrupted, divide them, as it were, into compartments, 

 within the wider flats and softer sloping banks of which 

 planation, by means of river-curves, is ceaselessly at work. 

 Each of these rock-gorges, as in the case just cited from 

 Morayshire, has checked the more rapid deepening natural to 

 the softer strata above and below it, and the activities of the 

 stream are spent in lateral movements. 



In countries where the glacial period has left thick deposits 



