RELATION TO MESOZOIC ROCKS IN THE LEPONTINE ALPS. 231 



with the unaided eye — a sample of the purest and most crystalline 

 condition of the rauchwacke ; the other from the matrix of the 

 basal breccia. 



I recognize the above-named minerals, for a careful description of 

 which I refer to Dr. Grubenmann's memoir. The structure, 

 however, of the rock, which does not appear to have impressed him, 

 appears to me of great importance. -Taking first the specimen 

 which is more crystalline in aspect, we find a minutely granular 

 matrix *, which, owing to the interstitial presence of a greyish dust, 

 is less clear and transparent than one would anticipate from the 

 macroscopic appearance of the rock. In this matrix are scattered 

 flakes of mica, mostly white, grains of calcite, of a colourless 

 mineral, often stained earthy brown, of quartz (here not common), 

 and of brownish or blackish iron-oxide. There is also one tiny flake 

 of a quartz-mica schist. Eut without the evidence of this last, which 

 is so small a fragment that I would not wish to rely upon its 

 testimonj', no one, I think, accustomed to the study of clastic and 

 metamorphic rocks could have any doubt that this rock cannot be 

 ranked with the latter group, that its matrix is not that of a true 

 marble, that the above-named minerals have been derived from older 

 rocks, and that these very probably were crystalline schists such as 

 have been described above. 



In the specimen taken from the breccia-bed (p. 209), the matrix 

 resembles that of the last one, but is slightly coarser and dirtier. It 

 is fuller of fragmental minerals ; both micas abound ; quartz is 

 common, indubitably in fragments, which are clear and give bright 

 polarization-colours like the grains in the schists ; calcite, iron-oxides, 

 &c. are present, with a few flakelets of mica-schist. These all are 

 beyond doubt derived from an older series, as in the other case. 



A specimen of the rauchwacke, containing fragments, from the 

 ravine at the north-west end of Lake Ilitom affords a matrix 

 generally similar to the above, with like mineral fragments, llutile 

 is rather abundant in parts of the slide, and frequently occurs in 

 <' nests," which, with the associated material, suggest the presence 

 of fragments. Be this as it may, chips of schist are undoubtedly 

 present, and I can hardly doubt that some represent the adjacent 

 Calc-mica series. 



A specimen also has been examined from the base of the rauch- 

 wacke in the second ravine, south-east of Lake Ritom (p. 206). It 

 was selected because it illustrated the rock in a crushed condition. 

 Even here, though the efl'ccts of pressure are very marked and the 

 rolling and crushing together of the matrix and included fragments, 

 sometimes of not very diflerent mineral composition, makes it more 

 difficult to follow the precise outline of the latter, still tlic calcite of 

 the matrix is but slightly more coarsely crystalline than in the 

 cases described above, the clastic character of the rock is not 

 destroyed, aiid it is very difl'erent from any specimen which I pos- 

 sess of the crystalline schists. Among the fragments, calc-mica schists, 



* The granules rarely exceed "001 inch, and are often less. 

 Q. J. G. S. :So. 182. R 



