RELATION TO MESOZOIC ROCKS IN THE LEPONTINE ALPS. 233 



possible that derivative flakes may subsequently have been slightly 

 enlarged. Now and then a slide contains a little brownish mica 

 which certainly has a derivative aspect. The larger minerals, 

 present, as it were, porphyritically, and giving the spotted character to 

 the rock, are certainly authigenous, and appear to have been formed 

 after the rock had assumed a slightly schistose structure under pres- 

 sure. At least three types can be recognized : — (a) The rounded 

 grains (Knoten). These are found on microscopic examination to con- 

 sist of the darker granular matter of the matrix with some quartz- 

 or calcite-grains (smaller than in it) cemented together by a clear, 

 rather wax-like material, which has occasionally separated itself 

 from the other constituents in thin streaks. It has weak double- 

 refraction, for with the two nicols its tints are low in the scale ; 

 certainly, as the analysis leads us to expect*, it is not garnet; but 

 it is in a condition so unfavourable for examination that I will not 

 venture to suggest an identification. Probably it is a hydrous 

 silicate of alumina and lime, possibly orthorhombic, but it does not 

 remind mo of snij mineral with which I am familiar, (b) The 

 prism-shaped minerals (Frismen), of which there may possibly be 

 two forms. The commoner are subangular to almost rounded 

 prisms, with surfaces rarely lustrous, but often spangled with 

 minute mica-flakes. These, in thin slices, exhibit a flaky-granular 

 mineral affording moderately bright polarization-tints, cementing 

 together a dusty-looking matrix (in this crystalline calcite cannot 

 be discerned). The outline of the prisms is rather irregular, but, 

 so far as one can judge, extinction takes place either parallel or at a 

 small angle with their sides. With this mineral I provisionally 

 include a somewhat shorter but decidedly more slender one (com- 

 moner, I think, on Scopi) which has a slightly fibrous aspect, with 

 indications of cleavages parallel with and oblique to the axis : 

 fractured surfaces having a rather resinous lustre. In thin slices, 

 though the outline is rather irregular, the crystals are more free 

 from foreign matter. The polari/ation-tints are rather low, whitish 

 or yellowish. Though extinction not seldom takes place parallel 

 with the sides (so far as they can be determined), it is, I think, 

 more often oblique than would occur with an orthorhombic mineral. 

 In thin sections some of the former prisms are very hard to dis- 

 tinguish from the first group, which seem sometimes to take an 



* Analysis of tlie minerals in the "Nufeneu Schiefer" (Fritsch, Beitriige zur 

 Geol. Karte cler Schweiz, xv. Lieferung, p. 127) : — 



(I.) In den (II.) In den 



Knoten. Prisnien. 



SiO^ SaOO 4007 



AlA 19-45 2205 



Fe;;0, 5-93 566 



CaO lOa^ 22-29 



MgO 1-03 1-20 



H^O 60() 8-89 



Loss 3-49 — 



10000 10016 



r2 



