KELATIOJSr TO MESOZOIC KOCKS IN THE LEPONTINE ALPS. 205 



retical difficulties that we are entitled to demand a very clear 

 demonstration of its stratigraphical position. What instance, which 

 has stood the test of careful examination, can be produced where a 

 bed, practically unmetamorphosed, is intercalated in a thick series 

 every member of which is highly crystalline ? Making every allow- 

 ance for differences of composition (though it might be observed 

 that highly crystalline calc-schists, limestones, and dolomites are 

 common enough among the supposed overlying series), we are 

 still compelled to ask. What process of selective metamorphism can 

 have produced such a result ? I put this question in the interests 

 of " a rational uniformitarianism." Accurate observation and 

 scientific induction lead us to believe that Nature acts in accordance 

 with laws, and that these are related to their results. This would 

 be an anomaly without precedent, and thus cannot be accepted 

 without very clear evidence. 



But when we come to study the rauchwacke more closely (con- 

 fining ourselves for the present to the Lepontine Alps) we find that 

 to regard it as the base of the Piora series (as is done by some of 

 the Swiss geologists) would lead to results which would be often 

 perplexing and sometimes self-contradictory. 



Por instance, in the Lukmauier-Piora district a brief study of the 

 map shows that the distribution of the deposit is very puzzling, and 

 our difficulties grow the more minutely we examine the terrain. 

 Below the mineral-bearing Jurassic beds of Scopi and Alp Vitgira the 

 rauchwacke is thin and sometimes wanting, but it occupies a consi- 

 derable area a little below the summit of the Lukmanier Pass, and 

 the group * soon attains a thickness which can hardly be less than 

 a hundred yards, and may probably be much more. This mass 

 (which seems as if embayed between Scopi and the hill of gneiss 

 facing it on the S.W. across the open basin traversed by the upper 

 part of the road) is an ofi'shoot from a broad strip which runs east- 

 ward to beyond the Yal Greina t, and westward towards the Val 

 Piora. The former part, I may remark in passing, is bordered on 

 the northern side by beds indubitably Jurassic, on the southern by 

 true schists X. The western strip of rauchwacke crosses a water- 

 shed, where, in the Pizzo Columbe, unless appearances are very 

 deceptive, it can hardly be less than 700 feet thick. Thence it 

 passes into the Val Piora. Here its distribution is very perplexing, 

 on the hypothesis of its superposition to the Piora schists. On the 

 northern side of the valley it appears to occur between these and 

 the Trcmola schists. West, and perhaps south of the lake it appears 

 to divide the former series in one part from mica-gneiss, in another 

 from ordinary gneiss; but the mass which extends westwards from 



* There is a thick bed of anhydrite or gypsum, 



t This joins the Val Levantina at Olivone. 



I This is the testimony of Dr. von Fritsch's map. Personally I can only 

 answer for the Lukmanier road, which in descending crosses black-garnet 

 schist, quartz-schist, and mica-schist with reddish garnet, staurolite, and kya- 

 nite — rocks which bear a general resemblance to those of the Val Piora. Lower 

 down, at the opening of the Val Greina, in the strike of the same mass, calc- 

 mica schists are abundantly developed. 



