Vol. 62.] TARNS OF THE CANTON" TICINO. 177 



animation. I found it no easy task to locate the exact dip and 

 horizon of these planes of movement : in fact, the beds have been 

 more crushed and rolled out than even Prof. Bonney himself, I think, 

 imagined. Not only are there undoubted lines of movement between 

 the crystalline rocks and the beds of rauchwacke, but, for some 

 distance from the schists, fragments of the latter rock have been 

 torn off and incorporated in the calcareous beds. In the lower part 

 of the schists again, between the two lakes, eyes of calcite and 

 dolomite have been rolled out and included in the crystalline schists ; 

 and it is possible to collect from the western end of La.go Ritom a 

 series of specimens, ranging from pure rauchwacke to a foliated rock 

 consisting of alternate layers of rauchwacke and schist. The whole 

 process must have been analogous to what I have seen taking place 

 in the case of advancing Arctic glaciers. Here not only are thrust- 

 planes formed by the continual retardation of the lower layers of 

 the ice, but, owing to the presence of shear-planes, fragments of the 

 underlying material are dragged out along them and arranged in a 

 roughly-stratified manner throughout the ice ; many of these are even 

 raised by this process for considerable distances above their source 

 of origin. 



We may, I think, then conclude that, so far as the valleys in which 

 the lakes lie are concerned, their formation has been deter- 

 mined by the presence of thrust-planes, and that, in the case 

 of the valleys in which the three larger lakes occur, the effect of these 

 thrusts was emphasized by the introduction of wedges of softer rock. 

 The overdeepened portions of the valleys in which the lakes occur 

 still, however, remain to be accounted for. They cannot, of course, 

 be due to ordinary mechanical erosion by the present stream ; but the 

 special character of the rocks seems to suggest that the chemical 

 action of water may have manifested itself in an unusual degree, 

 owing to the relatively-greater solubility of the calcareous rocks. 

 Analyses of three samples of the rauchwacke have kindly been made 

 for me at University College (London) by Miss Edith Goodyear, B.Sc. 

 These were collected from the barriers of rauchwacke rising from 

 the southern end of Lago Tom and the western end of Lago Ritom 

 respectively. I have added for comparison a fourth analysis, made 

 by Dr. Grubenmann, of the same dolomite where it crosses the Val 

 Canaria. 1 



I. II. III. IV. 



C0 2 39-21 39-30 41-88 44-96 



CaO 45-61 4541 41*90 40-12 



MgO 4-10 3-83 8-80 11-96 



rsio 2 , _ i-i9 — — 



LFe.,0 3 1-34 2-01 1-28 0-42FeO 



Insoluble residue 8-10 7"87 604 1-96 



Totals 98-36 99-61 99*90 *• 99-42 



No. II is from the broken-down underground channel described above. 

 It was especially selected, on account of the relatively-large amount of green 

 mica which it contained ; while No. I is an average sample of the rauchwacke. 

 No. Ill was collected from the cliff of rauchwacke cropping out on the east 

 side of Fongio. 



1 See Quart. Journ. Geol. Soc. vol. xlvi (1890) p. 230. 



