RELATION TO MESOZOIC KOCKS IN THE LEPONTINE ALPS. 229 



to minute details, it would be possible to indicate many varieties 

 both in the Val Canaria and in the Yal Piora sections, but I agree 

 with Dr. Grubenmann in regarding these as comparatively unim- 

 portant, except so far as they indicate (as we both agree in holding) 

 that the rocks are of sedimentary origin. The principal con- 

 stituents are calcite, quartz, mica, commonly white, but sometimes 

 brownish or greenish (from alteration) with black grains — iron- 

 oxides, and perhaps graphite — and more or less of dusty-looking 

 material, which Dr. Grubenmann considers to be of organic origin. 

 He also finds some zoisite, and rutile is occasionally present. The 

 quartz-grains, often about '012 inch to '025 inch in diameter, are 

 commonly very clear and give high polarization-tints. In no case 

 do they retain any trace of a clastic origin, though they often 

 appear to have been dislocated and again cemented. The mica 

 flakes are commonly about '03 inch long, and the calcite grains 

 about the same in diameter. Every specimen gives more or less an 

 indication of mechanical disturbance ; sometimes the pressure 

 appears to have acted at right angles to the mineral layers, some- 

 times there is evident shearing, sometimes it has acted more nearly 

 parallel with the layers, producing various flexures and foldings as 

 I described on a former occasion. Dr. Grubenmann himself states 

 that the mica flakes are often thrown into strong undulations 

 (sind oft stark luellir/ verhogen) and that the calcite shows con- 

 stantly twin structure parallel with — | B, and the twin lamellae 

 are frequently rather conspicuously bent. He remarks that this is 

 evidence of severe pressure, but does not seem to perceive the 

 necessary inference that the rock was alread)^ thoroughly crystal- 

 line when the pressure acted. 



So far as microscopic evidence goes, these rocks of the Yal Piora 

 group present in many cases close resemblances to members of the 

 " Upper schist group " (as defined by me) from other parts of the 

 Western, Central, and Eastern Alps, though, of course, varietal 

 differences are almost endless. Here dolomite may predominate 

 over calcite, here mica may be much more abundant, here there 

 may be a greater amount of this or that accidental constituent, but, 

 as it seems to me, " a family likeness,'' both macroscopical and 

 microscopical, is strongly marked, and the latter method of study 

 does but accentuate the difference between these and the so-called 

 " fossiliferous schists " of the Lukmanier and Nufenen Passes *. 



* Dr. Grubenmann gives the following as a bulk-analysis of atypical example 

 of the Calc-niica schist series of the Val Canaria, remarking that the rock must 

 consist of about three fourths of calcite, and the I'emainder quartz, clay, and 

 aorae mica : — 



SiO, 21-1)0 



Al,o, l'4a 



Fe^O, andFeO 0*78 



CaO" 4(V11 Sp. gr. =2-7674. 



MgO OGl 



CO, 3004 



loo-ya 



