260 BR. J. W. GRKGORY ON THE WALDENSIAN [Mil) 7 1 894, 



IV. Conclusions as to the Relations op the Beds. 



Prof. Bonney has recently insisted on the great difficulties in 

 the way of geological mapping in the Alps, 1 and though in the 

 Eastern Cottians there is not much difficulty from snow or glaciers, 

 the great extent of the moraines and vineyards in the valleys, and of 

 the pastures on the higher slopes, prevents any detailed mapping of 

 much of the area. The ruggedness of the country, the depth of the 

 valleys, and the lack of accommodation add further obstacles to the 

 investigation by causing so much of the time to be necessarily spent 

 in walking to one's field of work. 



It was impossible, in the short time available for field work, to 

 prepare any detailed map of the geology of the area, or even to 

 follow, as I had intended, the whole length of the junction of the 

 gneiss and the schists. This has been rendered less necessary by 

 the clearness of the sections at some points. A rough sketch-map 

 of the area is appended, on which the main facts have been inserted 

 (see fig. 1, p. 236), and it is perhaps advisable here to summarize 

 briefly the evidence afforded by the foregoing descriptions. 



As has been already remarked, according to the generally- 

 accepted theory, the * central gneiss ' is the lowest of a three-fold 

 series of Archaean I'ocks, and its position at the base of the series is 

 due to its being the oldest of the three, the others having been 

 deposited upon it. Against this view the evidence now presented 

 is fairly conclusive. It will bo generally admitted that if this be 

 true, a strong unconformity must occur between the schists and the 

 gneiss ; in support of this, it is only necessary to refer to the map of 

 Gastaldi reprinted on p. 240 and to his sections, such as that from 

 Roche Melon to Lanzo 2 or at Monte Resta 3 ; the section of Dr. 

 (iianotti 4 shows the same for the neighbourhood of Chialamberto. 



With such an unconformity it is incredible that no fragments or 

 pebbles of the gneiss should occur in the schists. The latter are a 

 very extensive series of deposits, and contain several beds of con- 

 glomerate, such as that at Tredici Laghi and under Punta Bruta. 

 But in no case do fragments of the ' central gneiss ' occur in them ; 

 a section of the Tredici Laghi conglomerate in the schist series shows 

 that the pebbles consist of a felspar-zoisite aggregate with numerous 

 crystals of rutile and some small flakes of biotite. These pebbles are 

 very dusty, and their outlines are rounded as if by crushing; they 

 are set in a matrix of a water-clear mylonitic mosaic of quartz and 

 felspar, which contains long blade-like crystals running round the 

 inclusions. The evidence of the slide is not conclusive, but the 

 rock appears more probably to have been formed by the crushing of 

 a felspar-conglomerate than of an augen-gueiss. 



At not one of the sections examined was there any evidence of 



1 Bonney, ' On the Crystalline Schists and their Relation to the Mesozoic 

 Rocks in the Lepontine Alps,' Quart. Journ. Geol. Soc. vol. xlvi. (1890) 

 pp. 187, 188. 



2 Gastaldi, ' Studii geologici sulle Alpi occidentali,' Mem. desoriz. Carta geol. 

 Italia, vol. ii. (1874) pi. i. f. 2. 



3 Ibid. vol. i (1871) pi. v. 4 Gianotti, op. jam cit. pi. v. 



