KELATION TO M1':S0Z0IC KOCKS IX THE LEPONTINE ALPS. 



203 



mica. On examining a cross-fracture with a lens many glittering 

 minute black particles are seen to be thickly interspersed with the 

 more micaceous constituents. Garnets are numerous ; they are 

 seldom more than one third of an inch in diameter, and commonly 

 about the size of peas. Though somewhat distorted, evidently 

 by pressure, they exhibit fairly definite crystal faces (dodecahedral) 

 and the usual aspect of a dark garnet when fractured ; but not un- 

 frequently they are traversed by a rude cleavage coincident with the 

 divisional planes of the rock, and obviously due to the same cause. 

 Their crystalline form is often better exposed on weathered surfaces, 

 and in this way sometimes fairly regular crystals may be obtained. 

 It is rather difficult to detach them from the matrix. The faces 

 are then lustrous, though corroded by the more rapid weathering 

 of the enclosed impurities, which are abundant. The ordinary colour 

 is black, but not seldom a slightly brown or reddish tinge may 

 be noticed, and occasionally they are of a deep claret-red. 



The folia in the matrix bend round them, and there is every indi- 

 cation that the rock has been subjected to severe pressure since they 

 were formed and it was foliated. This dark schist is associated with 

 layers of a light brown to greyish rather fine-grained quartzose 

 rock, of somewhat felspathic aspect, apparently resulting from the 

 alteration of a rather earthy sandstone. The one rock passes 

 rapidly into the other ; sometimes the two are closely interbanded, 

 laminae of black schist alternating repeatedly with those of quart- 

 zite, the thickness of either varying from that of a stout card up to 

 about half an inch, now one now the other predominating ; but 

 sometimes the rocks occur in much thicker masses. Garnets are 

 common in the dark layers, sometimes bulging out a little beyond 

 the surface of a thin one, but are very rare and small in the 

 (juartzose layers. The pressure to which the cleavage-foliation is 

 due has acted commonly (in the Val Piora) at right angles to this 

 lamination, but there are occasional exceptions to the rule, and 

 the rock then exhibits contortions. 



Some details of the microscopic structures of these Piora schists 

 are given in the Appendix (p. 224). It may suffice to say here 

 that, after careful study of the rocks of this group in the field and 

 with the microscope, the following conclusions appear to me fully 

 established : — 



1 . This group was once a series of more or less banded sediments, 

 not unlike some of our Coal-measure shales and sandstones. 



2. By some agency these were completely metamorphosed, all 

 traces of the clastic structure of the sandstones being obliterated, 

 and a number of new minerals formed, among them (in the black 

 bands) garnets of considerable size. 



3. The whole mass was subsequently exposed to severe pressure, 

 doubtless during one or more of the processes of mountain-making, 

 which has produced tlic usual effects. 



4. There is no evidence whatever to show that a pressure definite 

 in direction had anything to do witli tlie production of the garnets. 

 So far as we can tell, tliey, as is usual with the garnets in the Alpine 



