176 PROF. E. J. GAEAVOOD 0]ST THE [May I906, 



Lastly, if the ice came from the north, how was the Lago Scuro 

 excavated, or the Lago Taneda, which is entirely shut in by pre- 

 cipices on the north ? It is, indeed, extremely difficult to account for 

 the excavation of these last-named tarns by ice, even if we attribute 

 ^0 that agent the utmost power that has ever been claimed for it. 



Let us take the Lago Scuro as a crucial case. Here is a rock- 

 basin 136 feet deep, situated almost on a main watershed, within 

 a few metres of its summit, and surrounded on three sides by steep 

 cliffs. There is no gathering-ground above it for a glacier, and 

 it is thoroughly protected from any ice invading the region from 

 outside : yet we find a rock-basin of this considerable depth. I need 

 hardly add that the steep reefs of rock running into the lake are 

 quite inconsistent with ice-erosion. The case of Lago Taneda is still 

 more difficult. No ice could possibly reach this lake from anywhere 

 except a little neve from Taneda, and yet it is a deep rock-basin. 



Geological Structure. — Putting aside, then, the theory of 

 ice-erosion, we must seek for some other reason for the existence of 

 these lakes. 



Let us first of all consider, in rather more detail than we have yet 

 done, the geological structure of the district. 



All the rock-basins of this group, as shown above, are situated 

 along the junction of two distinct types of rock. The three lower 

 lakes lie along the strike of two nearly-parallel outcrops of rauch- 

 wacke and dolomite, and occur at the junction of these rocks with 

 gneiss and crystalline schists. 



A glance at Dr. K. von Pritseh's map shows, not only the lower 

 lakes, but also the valley of Piora running along the outcrop of the 

 rauchwacke and dolomite, while the same rocks occupy the two 

 passes leading into the Val Canaria between Pongio, Pian' Alto, and 

 the Cima di Camoghe. We have, therefore, in this district 

 differential erosion depending on differences in geo- 

 logical structure, being greatest along the rocks rich 

 in calcite and dolomite. In addition to this, the 

 soundings show that the axes of greatest depth of all 

 the lakes lie along the junction of calcareous and 

 crystalline rocks. 



The causes which produced the peculiar geological structure of the 

 district still remain to be considered. In his well-known paper, 

 ' On the Crystalline Schists & their Relation to the Mesozoic Eocks 

 in the Lepontine Alps', 1 Prof. Bonney has argued against the 

 inclusion of the rauchwacke and dolomite in the same metamorphic 

 series as the crystalline schists occurring above and between them. 

 He regards them as an entirely-separate group of rocks, introduced 

 among the crystalline series by faults or thrusts, and rejects the 

 idea suggested in the sections accompanying Dr. K. von Pritseh's 

 map, that their present relation to the schists is the result of simple 

 folds, which affected a stratigraphically-continuous series of beds. 



After my first visit to the district, and before I had seen Prof. 

 Bonney's paper, I had arrived at precisely the same conclusion; 

 a conclusion which has been fully corroborated by subsequent ex- 



1 Quart. Journ. Geol. Soc. vol. xlvi (1890) p. 187. 



