226 



PROr. T. G. BONNEY ON CRYSTALLI^s^E SCHISTS AND THEIR 



The " quartzite " interbanded with the dark schist consists, in the 

 specimen which I have examined *, chiefly of two minerals in 

 variable proportions. The one is quartz, in grains (often about '01 

 inch in diameter) of roundish but slightly irregular outline, like 

 those common in many quartzose schists, except that they are rather 

 full of microlithic enclosures and fluid-cavities. The other is a 

 mineral, which at times is rather earthy-looking, like a decomposed 

 felspar, at times is clear with a rather strong double refraction. 

 The former seems to pass into, as if rej^laced by, the latter. This 

 occasionally occurs in rude prisms, and presents some resemblance 

 to an impure andalusite, having in one or two instances the brownish 

 tint of staurolite. Their form is hardly regular enough to determine 

 satisfactorily the extinction-angle, but I venture to suggest that 

 they are a mineral allied to andalusite, and have been produced 

 from a decomposed felspar. Zircon (?) and rutile occur occasionally. 

 One of my slides exhibits a close interlamination of this rock and of 

 the dark schists. Here the former contains a few grains which 

 may be small reddish garnets, and the latter exhibits the tourma- 

 line-like mineral in a form that suggests a replacement of the 

 brown mica. 



The Staurolite-schist from near the TI.E. end of Lake Eitom has 

 as its matrix a closely felted mass of white mica and quartz (both 

 minerals being rather minute) in which are scattered grains of iron- 

 oxide, flakes of brown mica, reddish garnets of fair size, and the 

 large staurolites. The last mineral is a lioney-brown colour in 

 thin slices. It contains numerous microliths which I will not 

 venture to identify, but flakes of an iron-oxide (? haematite) are not 

 rare, and prisms of rutile are fairly abundant. The garnet here is 

 " cleaner "' than in the black schist, still enclosures are plentiful. 

 The outline of the staurolites is sometimes fairly rectilinear, but 

 sometimes rather irregular and broken -looking. On the whole the 

 evidence seems favourable to the existence of this mineral anterior 

 to the date of the mechanical disturbance which has affected the 

 group. 



A schist, with staurolites of smaller size, in a rather more 

 micaceous matrix, occurs on the south side of the Lukmanier Pass, 

 above Somascona. It appears to have been subjected to pressure 

 after it had attaiiied to its present mineral conditions t. 



To the S.E.- of the Lago de Ilitom, appEirently on nearly the 

 same horizon as the Staurolite-sehist described above, are outcrops 

 of rock containing kyanites, which sometimes are full 3 inches in 

 length, in a rather gneissose ground-mass. This, however, does 

 not caU for any special notice, because smaller kyanites are far from 

 rare in the more micaceous bands of the Calc-mica-schist group. 

 Kyanitc occurs also plentifully on the south side of the Lukmanier 



* From the ravine at the east end of the Ritom Boden. It seemed needless 

 to liaAC other specimens sliced. 



t For a brief description of the group of rocks with which this schist is 

 connected, and which evidently is a prolongation eastward of the Piora group, 

 see my President's Address, 188G, Quart. Journ. Greol. Soc. vol. xhi. Proc. p. 47. 



