574 



MISS M. I. GAEDINER ON CONTACT-ALTERATION 



signs of contortioD. Fig. 2 represents a piece of weathered rock 

 in cross section. Quartzose layers, of one of wliicli the little 

 lenticle at the top of fig. 7, PI. XXIII., is a section, form the 

 prominent bands ; and lines of brown mica weather out, leaving 

 fine ridges, conforming to the twists and turns of these bands. 



Pig. 2. — Altered Grits contorted. 



/ Foo^ 3 Inches. 



Weathered surfaces of this kind are common and striking ; but 

 perhaps, considering how quartzose it is, the character of the rock, 

 which is exposed just below the one E. and W. wall along the hill- 

 side, is more so. A vertical section shows Ys, of about \ inch in 

 length, through whose points the rock cleaves horizontally or nearly 

 so. This rock (fig. 8, PL XXIII.) consists of the same minerals as 

 the other altered grits, together with a little chiastolite. The Ys 

 are formed with thick bands of light brown mica-flakes, always 

 well-formed and never bent. The cleavage takes place along bands 

 of quartz-grains, which run through the points of the Ys. This 

 quartz consists of small granules, such as form the quartzite-like 

 base of the other altered grits. Chiastolite is seen in the two 

 rectangular sections on the left of the slide (fig. 8, PL XXIII.). 

 The light patch at the top, where it is clearly marked by black 

 lines, consists of large flakes of white mica, probably replacing 

 cbiastolite. It will be noticed that the chiastolite affects the folds, 

 and that the garnets often lie in small lenticles of quartz. 



following the grit bosses up the hill, at about 200 yards from 

 the granite junction, the lower cairn (see Map, fig. 1) is reached. 

 The rock attracts attention by the brilliant sparkle of the small 

 white mica-flakes. A cross fracture shows the rock to resemble 

 the more quartzose varieties of the altered rocks just described, but 

 to be rather coarser in structure, and to contain more white mica. 

 The mica-flakes are about 4-js ii^ch in diameter. Though, roughly 

 speaking, their cleavage-faces are parallel, they, are not in suffi- 

 ciently continuous planes to make the rock cleave easily. The 

 rock still has the purplish tint of the grits in the stream. 



The microscope shows that the rock varies in texture in difierent 



