^■.\...,^,, .i^Formation of' Lake-Terraces. 31 



clined as the bank above. The terrace consisted of grains ot 

 sand and small pebbles, mixed with red clay. 



Having been struck with the miniature resemblance of this 

 terrace to the parallel roads of Glen Roy, as described by Dr 

 Macculloch, and having reflected upon the way in which the 

 rain-water had produced it, I was soon led to form a general 

 conclusion respecting the conveyance of all kinds of sediment 

 by running into standing water, viz. that, if a b d. Fig. 23, in- 

 stead of representing merely the section of an artificial bank of 

 gravel, represent the side of a hill, a mountain, or any other de- 

 clivity, descending into a pool, lake, or any other piece of stand- 

 ing water, and, if water flow along a b, carrying earthy mate- 

 rials of different degrees of fineness, its course will be changed 

 at the point b, where it meets the standing water ; that it will 

 diff'use itself over the surface of that standing water, carrying 

 with it the finest particles ; that all the particles, which are too 

 large and heavy to be held in suspension, will I'all by their own 

 w^eight ; that, by the continuance of this process, a terrace will 

 be formed, having a gently inclined surface b c, over which the 

 stream will continue to roll and shove the coarser detritus ; and 

 another surface cd, inclined at an angle of about 45°, down which 

 that detritus will fall, as it would in air ; and that the line c d 

 will continually advance by the accession of fresh materials, pre- 

 serving always the same inclination until the pool is filled. 



In several of the Swiss lakes I had an opportunity of finding 

 this view confirmed. For example, in the Lungernsee, Canton 

 of Unterwalden, nearly the whole margin is accompanied by a 

 terrace, which varies in breadth from 5 to 12 or 14 feet, sloping 

 very gradually under the water, and then terminating in a steep 

 declivity. The terrace is easily distinguished, even at a dis- 

 tance, by the brown colour of its stones, contrasted with the 

 green of the deep water. In some parts its extent is equally 

 well defined by the reeds, rushes, and water-lilies, which grow 

 upon it, but do not pass beyond tiie edge of its steep declivity. 

 According to the observations of Dr Macculloch * and Sir Tho- 

 mas Lauder j-, the mountain-lakes in Scotland and Italy, as 

 well as in Switzerland, have similar terraces along their shores. 



• Trans, of Geological Society, vol.. iv. p. 309, 370. 



t Trans, of Royal Society of Edinburgh, vol. ix. p. IC, I7. 



