TERRACED SLOPES IN NORTH TYNEDALE. 39 



emergence of the Newer Pliocene strata," gives a striking in- 

 stance of this configuration in connection with the same moun- 

 tain limestone formation which prevails here ; and he further 

 illustrates his subject by an engraving. He says, " In the lime- 

 stone districts of the Val di Noto (in Sicily) the strata are for the 

 most part horizontal, and on each side of the valley form a suc- 

 cession of ledges or small terraces, instead of descending in a 

 gradual slope towards the river plain in the manner of the ar- 

 gillaceous formations. When there is a bend in the valley the 

 exact appearance of an amphitheatre, with a range of marble 

 seats, is produced."* Something slightly similar to this pic- 

 turesque appearance has been remarked in connection with the 

 Swinburne terraces in this valley. When these long level es- 

 carpments occur near to the present channel of rivers, of which 

 every one has seen an example, it is easily perceived how they 

 denote changes, often recurring in the river bed itself by the 

 disintegrating action of winds and rain, and the erosive force 

 of floods. It is, however, in cases like those now under consi- 

 deration, where the terrace-lines are found on the higher slopes 

 of a valley or glen, that we pass from a simple to a more com- 

 plicated phase of the subject, and from comparatively modern 

 times to the indefinite remoteness of the later geological periods. 

 This has been especially the case with the wonderful terraces or 

 " Parallel Eoads of Glen Roy." Sir C. Lyell, in his "Antiquity 

 of Man," t treats them at considerable length, and concludes 

 that these and the terrace-lines of some neighbouring valleys 

 "were formed on the borders of glacier lakes," that the vales 

 themselves then formed shallow lakes, whose level is marked by 

 the uppermost shelves, the escape of water over any "col," or 

 parting ridge of lower level between the glens, being prevented 

 by the protrusion of a glacier from above so as to rest like a 

 barrier on the flank of the hill ; and the lower shelves or terraces 

 would afterwards be formed by the shrinking of the ice into less 

 dimensions, and the escape of the waters of the lakes over and 



* " Principles of Geology," 4tli Edition, Vol. IV., p. 7. 

 t ^nJ Edition, Class XIII., pp. 252 to 2G4. 



