206 



PROP. T. G. BONNEY ON CRYSTALLINE SCHISTS AND THEIR 



the Pizzo Columbe narrows and disappears in the michlle of the 

 Piora schists. This, with the occurrence of one or two lenticular 

 patches in like position, can onl}' be explained, if the above h\'po- 

 thesis be correct, by verj^ peculiar rolling and faulting. 



]Srot less perplexing is the occurrence of another irregular isolated 

 and probably interrupted strip on either side of the Pian Alto, west 

 of the lower end of the Lake Eitom. 



Our difficulties increase as we examine the sections in detail. 

 Assuming the rauchwacke to be at the base of the Piora schists, let 

 us direct our attention to two sections in ravines on the south side 

 of the Ritom Boden. If we climb up the ravine traversed by the 

 second stream (counting from the west) we find the rauchwacke 

 resting upon gneiss. The upper part of the latter roclt is tremend- 

 ously crushed ; its quartz-veins are rolled out and smaslied ; it is 

 converted into a fissile silvery schist ; above it comes a foot or two 

 of rock, which is practically past identification. It may be crushed 

 gneiss, friction- or basement-breccia, for it has been smashed beyond 

 recognition. Then comes the rauchwacke, slabby and rather schis- 

 tose. Here and there it contains a few fragments ; but these, so 

 far as I saw, were gneiss, so they prove no more than that the rock 

 is much later than the Lepontine gneiss (which all admit) and is 

 not highly metamorphosed. Here, then, the rauchwacke either rests 

 directly upon the gneiss or is brought against it by a thrust-fault. 

 Let us now pass on to the next ravine, which for a considerable 

 distance runs almost along the other edge of the patch of rauch- 

 wacke. In the lower part we find the following section, not, indeed, 

 continuously exposed, but indicated by crags and ridges outcropping 

 from turf or debris (fig. 2) : — (1) Dark-mica schist (well displayed) ; 



JFig. 2. — Section in lower part of Ravine, south-east of 

 Lake Ritom. 



A. Dark-mica scliist. C. Calc-iuica schist. R. Rauchwacke. 



The part left blank is covered ground. 



(2) Calc-mica schist, parted by the stream from (3) rauchwacke. 

 But about 400 feet above the lake we find (fig. 3) ; — (1) Dark-mica 



