1026 REPORT—-1885. 
these views appear to agree with those worked out independently by Messrs, Peach 
and Horne in 1884. 
Part IT, 
In the second part of his paper the author gave a summary of the work accom- 
plished among the metamorphic rocks of the Alps and Eastern Germany by Heim 
and Lehmann, and described the several types of rock metamorphism found in the 
Eriboll district, as worked out by himself. 
The Arnaboll (Hebridean gneiss), can be traced stage by stage from spots where 
it retains its original strike and petrological characters, to others where it acquires 
the normal strike and mineralogical features of the ordinary Sutherland schists. 
The old planes of schistosity become obliterated and new ones are formed; the 
original crystals are crushed and spread out, and new secondary minerals (mica 
and quartz) are developed. The most intense mechanical metamorphism occurs 
along the grand dislocation (thrust) planes. The gneisses and pegmatites resting 
on that plane are crushed, dragged, and ground out into a finely laminated 
schist (mylonite; Gr. mylon, a mill), composed of shattered fragments of the 
original crystals of the rock, set in a polarising cement of secondary quartz, the 
lamination being defined by minute inosculating lines (fluxion lines) of kaolin or 
chloritic material, with secondary crystals of a micaceous mineral. Whatever rock 
rests immediately upon the thrust-plane, whether Archean, Igneous, or Paleozoic, 
&c., is similarly treated, the resulting mylonite varying in colour and composition 
according to the material from which it is formed. 
The variegated schists, which form the transitional zones hetween the Arnaboll 
gneiss and Sutherland mica-schists, are all essentially mylonites in origin and 
structure, and appear to have been formed along great dislocation planes, some of 
which still show between them patches of recognisable Archean and Paleozoic 
rocks. These variegated schists (phyllites or mylonites), differ locally in composi- 
tion according to the material from which they have been derived, and in petrolo~ 
gical character according to the special physical accidents to which they have heen 
subjected since their date of origin, forming frilled schists, veined schists, glazed 
schists, &e. &e. 
The more highly crystalline flaggy mica-schists &c. which le immediately to 
the east of the zones of the variegated schists, appear to have been made out of 
similar materials to those of the variegated schists, but to have been formed under 
somewhat different conditions. ‘They show the fluxion-structure of the mylonites, 
but while the differential motion of the component particles seems to have been 
much less, the chemical change was much greater. In some of these crystalline 
schists (the augen-schists), the larger crystals of the original rock from which the 
schist was formed are still individually recognisable, while the matrix now con- 
taining them is a secondary crystalline matrix of quartz and mica arranged in the 
fluxion-planes. While the mylonites may he described as microscopic pressure- 
breccias with fluxion-structure, in which the interstitial siliceous and kaolinitic 
paste has only crystallised in part; the augen-schists may be described as pressure- 
breccias with fluxion-structure, in which the whole of the interstitial paste has 
crystallised out. The mylonites were formed along the thrust-planes, where the 
two superposed rock-systems moved over each other as solid masses, the augen- 
schists were probably formed in the more central parts of the moving system, 
where the all-surrounding pressure forced the rock to yield somewhat like a plastic 
body. Between these augen-schists there appears to be every gradation, on the 
one hand to the mylonites, and on the other to the typical mica-schists composed 
of quartz and mica. 
Like the mylonites, the crystalline augenites and micalites present us with 
local differences in chemical composition (calcareous, hornblendic, quartzose, &c.) 
They also show corresponding structural varieties due to secondary changes (frilled, 
veined, glazed, &c.), as well as others due to the presence of special minerals 
(garnet, actinolite, &c. &c.). 
