202 



PROF. T. Gt. EONNEY ON CKYSTALLINE SCHISTS AND THEIR 



fault ; but on this matter I have not been able to come to a conclu- 

 sion, and I regard it as subordinate in importance. 



This Piora series, as already said, is very variable in mineral 

 character. As I have previously noticed some of its members *, and 

 as it has been described in this or that locality by Dr. von Fritsch, 

 Dr. Grubenmann, and others, I shall pass very briefly over the macro- 

 scopic characters of all but the Dark-mica and Black-garnet schists, 

 which alone have a direct relation to the main subject of this paper. 

 SufHce it, then, to say that all the members of the Calc-mica-schist 

 group are distinctly (and, if uncrushed, moderately coarsely) crystal- 

 line ; and that they almost invariably exhibit a cleavage-foliation 

 which often is very marked. Sometimes it seems to have obliterated 

 the original "stratification-foliation,'' sometimes it coincides with it, 

 sometimes it crosses it at high angles. The evidence for all this 

 has been so often stated in detail by myself that I need not weary 

 the reader by repeating it t. White mica is common, and generally 

 glazes the "sheen surface;" but some specimens have a fair amount of 

 greenish or brown mica. Light- colon red quartz-mica schists and a 

 rather soft schist with two micas are found ; but, as said above, in this 

 district the rock is more commonly calcareous, varying from calc- 

 mica schist to white slightly micaceous marble. Eocks with iden- 

 tical lithological characters occur at intervals from one end of the 

 Alps to the other, and are connected by other schists which exhibit 

 only varietal differences. 



On the staurolite- and disthene-schists it is needless to dwell, for . 

 these also have been often described. It ma}' suffice to say that I 

 have examined microscopically specimens of both, and that they are 

 true crystalline schists. Once, no doubt, they were sediments ; but 

 all trace of their original clastic structure has been obliterated. 

 The crystals of staurolite and disthene are sometimes as much as a 

 couple of inches in length, though this, so far as I have seen, is an 

 exceptional size. 



The Dark-mica schist, however, with the Black-garnet variety,' 

 demands a longer notice, because it is through these that a link is 

 sought with the fossiliferous Jurassic rocks. It will suffice, however, 

 to describe the latter schist minutely, because the description, when 

 the garnets are omitted, will apply, in general terms, to the former. 

 The Dark-mica schist with black garnets is rather fine-grained, of a 

 very dark colour and graphitic aspect. The matrix, especially on 

 slightly weathered surfaces, exhibits a fairly, sometimes very, distinct 

 foliation, though the mineral constituents do not appear to be large. 

 Though the rock in cross-fracture has a strong tough aspect, resem- 

 bling that of one of the harder schists, it has a more or less fissile 

 structure, doubtless the result of pressure subsequent to foliation, 

 and the " sheen surfaces " exhibit a distinct lustre, which varies 

 from one not very unlike that of •' black lead " to that of a silvery 



* Anniversary Address to the Geol. Soc. (Proc. Geol. Soe. vol. xlii. (18S6), 

 pp. 45-47). 



t Quart. Journ. Geol. Soc. vol. xlii., Proc. pp. 36, 37, 69 ; see also vol. xlv. 

 pp. 82 &c. 



