224 prof. t. g. bonxey on crystalline schists and their 



6. Appendix. 

 (a) Microscopic Structure of the " Val Piora " ScJiists. 



The macroscopic aspect of this group has been described above ; 

 the microscopic structure of such members of it as appear in the 

 Val Canaria Section (where most of the characteristic varieties can 

 be recognized) has been studied by Dr. Grubenmann. As I have 

 nothing important to add to his excellent descriptions of the mineral 

 constituents, I shall content myself with quoting his results, in- 

 corporating with them some of my notes on specimens from the 

 Yal Piora and on the significance of the rock structures, a point to 

 which Dr. Grubenmann appears to have devoted less attention than 

 to their mineral composition *. 



The Dark-mica schists, including those containing blackish garnets, 

 are associated, as has been said, with bands of brownish quartzite of 

 variable thickness and persistency. The dark schists consist of 

 quartz, calcite, and two micas, with various other minerals, of which, 

 in certain cases, garnet is the most conspicuous. Regarding those 

 first named as the more essential components of the rock, we find 

 that the ratio of the first two varies much, the calcite sometimes 

 predominating over the quartz, while sometimes it is almost absent ; 

 dolomite also is probably present in some cases. The quartz is 

 clear, occurring in subpolygonal or rounded grains, and does not 

 retain any record of clastic origin. These grains, to some extent, 

 and still more those of calcite, are interspersed, and sometimes even 

 crowded, with a minutely granular blackish to greyish mineral 

 (probably in part graphite). Of the two micas, which occur in 

 fiakes usually '01 to -02 inch long, one is colourless. This is identified 

 by Dr. Grubenmann with margarite. The other is brown, dichroic, 

 giving a rich brown for vibrations parallel with the basal cleavage, 

 and a pale straw- or greyish green for those perpendicular, and is 

 considered to be probably biotite. The garnets in thin slices 

 appear of a very faint reddish colour, but the included " opacite " 

 often renders them almost opaque. Pressure, subsequent to crys- 

 tallization, has set its mark upon them. Thej^ are cracked or 

 rudely cleaved ; they are slightly distorted in form, or even some- 

 what irregular in outline ; occasionally their edges are quite 

 irregular, and the mineral passes into a fine-grained granular 

 mixture of quartz and garnet t. The adjacent matrix is bent and 

 disturbed by the resistance which they have offered; frequently it 

 has gaped open a little " under the lee" of the garnet as the pres- 

 sure again diminished, and the space has been filled up subsequently 

 by secondary quartz, with perhaps some calcite or a flake or two of 



* We have, in fact, approached the subject by different paths. I do not 

 pretend to have investigated the mineral composition of these rocks so elaborately 

 as he lias done ; for the history of a rock is to me of more interest than the 

 evhauslive study of its mineral details ; so that I am often content to leaA'e 

 (piestions relating to these unsettled. I find also that the use of high-power 

 lenses and an^' delicate optical work proves very trying to my eyes, which 

 are far from strong. 



<■ See page 23"). 



