200 



PROF. T. G. J30NNEY ON CRYSTALLINE SCHISTS AND THEIR 



I have been able to ascertain, these, though occasionally very abun- 

 dant, are rather impersistent in their occurrence. Apparently under- 

 lying these is a zone which, were it not still less persistent and 

 much thinner, one would be disposed to regard as another subgroup ; 

 this is characterized by the presence sometimes of large staurolites, 

 sometimes of kyanite (disthene). 



The second subgroup consists of a number of calcareous and mica- 

 ceous schists, occasionally quartzose, generally light in colour, fre- 

 (lueutly varying from calc-mica schists to a slightly micaceous 

 marble, the latter usually of no great thickness. I will refer to the 

 first as the Dark-mica schists (" Graue oder Biindner-schiefer in 

 (xlimmerschiefer iibergehend," von Fritsch), the garnet-bearing 

 variety as Black-garnet schists* (" Granatfiihrende schwarze Schiefer, 

 ■id.), the next as Disthene schists ('' Disthen und Staurolith fiihrende 

 8chiefer," id.), and the last as the Calc-mica schists (" Kalkglimmer 

 Schiefer," id.). 



After examination of these rocks both in the field and with the 

 microscope the following conclusions appear to be beyond any 

 doubt: — (1) That the series originally consisted, mainly if not 

 entirely, of sedimentary deposits ; (2) that its members are now 

 truly crystalline and generally rather coarsely crystalline ; (3) that 



Eig. 1. — Section (diagrammatic), luith interpretation, across 

 Lake Ritom and the Val Piora. 



Gr. Gneiss &c. A. Dark -mica schist &c. 



C. Calc-mica schist. F. Faults. 



B. Staurolite schist. 

 (1). Lake Ritom. 



after they had become foliated rocks they were subjected to pres- 

 sure, often great, which very commonly, but by no means always, 

 has acted at right angles to the original foliation — that is, these 

 rocks exhibit a cleavage-foliation as well as a stratification-foliation, 

 though very often the two are coincident in direction. 



What, then, is the geological position of these rocks ? The annexed 

 section (fig. 1) exhibits their mode of occurrence in the neighbour- 



* I formerly spoke of it as melanite-schist, but from a partial analysis made 

 by Dr. Grubenraanu (Mitth. der Tburg. naturf. Gesellscb. Heft viii. 1888) 

 the mineral appears to be an alumina-lime garnet, its blackness being clue to 

 in])urities which prevented any precise determination. 



