180 PKOE. E. J. GAKWOOD ON" THE [May I906, 



depths. They find that anomalies of temperature occur, similar to 

 those discovered by M. Yenukoff in the Black Sea. "With a 

 surface-temperature of 13'2° C. a gradual decrease takes place until 

 a depth of 33 feet is reached, where a minimum temperature of 

 5'1° is found. Below this, the temperature gradually increases 

 until the bottom is reached, where, at a depth of 144 feet, the 

 temperature is found to be 6'6° C. This rise of temperature in 

 the bottom-waters of the lake appears to me to point to under- 

 ground springs as the cause of concentration of salts in the lower 

 layers, rather than to the sinking of concentrated surface-water in 

 winter. 



Another interesting point is brought out by the presence of a 

 large quantity of sulphuric acid in the deepest layer of Lago Eitom. 

 The authors attribute the presence of this acid to the solution of 

 gypsum, but there is no bed of gypsum recorded among the rocks 

 which crop out round the lake. It will be seen, however, that the 

 bed of rauchwacke shown on Dr. K. von Fritsch's map, as running 

 from Pian' Alto into the Val Oanaria, includes large beds of 

 gypsum where it thickens out to the west ; and that, if Lago Eitom 

 occupies the site of a similar ' eye ' of rauchwacke, it is highly 

 probable that this eye includes beds of gypsum as well as rauch- 

 wacke, for it lies along the same line of strike. In this connection, 

 the total absence of sulphuric acid from the waters of Lago Tom 

 is a highly-suggestive fact. 



The analyses, then, of MM. Delebecque & Bourcart seem to 

 confirm the idea that Lago Eitom owes its origin, in part at all 

 events, to solution.] 



Although the presence of these lakes appears to depend chiefly 

 on the structural peculiarities described above, accompanied, in 

 some cases at all events, by solution, there is one remarkable 

 feature in the drainage of the district on which I have only briefly 

 touched : I refer to the way in which Lago Eitom is surrounded 

 by hanging valleys. Not only is this the case with all the three 

 streams flowing into it, but it is still more marked in the case of the 

 La Poos torrent, which flows out of the lake, and carries away the 

 whole of the drainage of the Yal Piora. This torrent plunges by a 

 series of cascades 2700 feet down into the Ticino Eiver, at an average 

 slope of 1 in 2 - 6. As I have shown elsewhere, this torrent forms 

 one of a series which drain from hanging valleys into the Ticino as 

 a consequence of the overdeepening of the main valley. While 

 other valleys, such as the Yal Canaria, have adjusted themselves to 

 the Ticino, the Yal Piora has been prevented from doing so by the 

 presence of the lake, which has arrested erosion by holding up 

 the solid particles on its floor. 



Let us trace the history of the Piora drainage a step farther 

 back. 



A glance at a model or map of the district shows that La Poos 

 is obviously not the natural outlet of the present Yal-Piora drainage : 

 it is evidently either a ' stolen ' outlet, due to some disturbance in 



