264 



Mr. W. H. Pearsall. 



also be remembered that the softer rocks along the shores of the more silted 

 lakes are more easily eroded by waves. 



Another criterion of the silting conditions in any lake may be employed. 

 Since suspended matter makes water more opaque, the relative transparency 

 of the lake waters may be used as an indication of the quantity of suspended 

 matter they contain. The basis of the comparison is taken as the greatest 

 depth at which a white disc (7 cm. in diameter) can be seen. The results are 

 contained in the third column of Table I, and they show that the clearest 

 waters occur in those lakes with rocky shores. The test must be used with 

 caution, for some of the lake waters are coloured with dissolved peaty matter, 

 ■c.(j., Bassenthwaite and Esthwaite. But it is permissible to conclude from 

 these results that there are greater quantities of suspended matter in the 

 waters of those lakes with silted shores. Moreover, since only the finest 

 matter can remain long in suspension, the greater opacity of the waters of 

 these lakes indicates that their affluents bring in material in a finer state of 

 siibdivision than that carried into rocky lakes. It has already been shown that 

 this is a priori probable. Further evidence in support of this assumption is 

 obtained by comparing the mechanical analyses of muds taken from Wast- 

 water, the most rocky lake, with those taken from Esthwaite, the most silted 

 lake. The following results are then obtained : — 



Table II, — Mud Analyses. 



Material. 



Wastwater (rocky). 

 Per cent. 



Estliwaite (silted). 

 Per cent. 





75-1 



82 -8 



74 -9 



62 -0 



54-2 



34 -2 



28 -2 



Silt 



14-8 



15-0 



16-1 



17 -6 



14-6 



29 -3 



25 -3 



Fine silt and clay 



10-1 



2-2 



9-0 



20 -4 



31-2 



36 -5 



46 -5 





1 



2 



3 



4 



5 



6 



7 



The muds for these analyses were all obtained from depths of 8 to 12 feet 

 (2-4 to 3'6 m.), so that they are fairly representative. The varying forces of 

 wave action make an exact comparison very difficult, but the differences are 

 sufficiently striking to make it probable that the sediments of rocky lakes tend 

 to be coarse, while those of silted lakes tend to be finer. Field notes are quite 

 in agreement with this conclusion. 



Thus on their physical characters alone the lakes can be divided into two 

 extreme types, rocky and silted. Wastwater, Ennerdale, Buttermere, and 

 Crummock represent the rocky type, while Coniston, Windermere, Ullswater, 

 and especially Esthwaite, belong to the silted type. Derwentwater, 



