580 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. 
These strata are ranged in steep anticlinal and synclinal folds which run 
across the south of Scotland in a general north-east and south-west 
direction. It is observable that this normal strike continues, with little 
modification, up to the granite, which thus has replaced an equivalent 
area of sedimentary rock (see p. 542). ‘The coarser arenaceous beds, as 
they approach the granite, are changed into quartz-rock, the thin siliceous 
shales into Lydian-stone, the black anthracitic graptolite-shales into a 
compact mass charged with pyrites, and breaking into large rough blocks. 
Strata wherein felspar-grains abound have been altered to a greater 
distance than the more siliceous beds, and show a gradation through 
spotted schists, with an increasing development of mica and foliation, 
until along the edge of the granite they become true mica-schist and 
even a fine kind of gneiss.} 
Closely analogous to these examples are those described by Fuchs? 
from the French Pyrenees, and by Rosenbusch? from the Eastern 
Vosges. In the former case the metamorphism of clay-slate is traced 
through spotted schists (Frucht-, chiastolite-, and andalusite-schists) into 
mica-schist and gneiss. In the latter a zone of alteration is shown 
to surround the granite boss of Barr-Andlau. The unaltered clay-slates 
are grey, brown, violet, or black, thinly fissile, here and there curved, 
crumpled, and crowded with kernels and strings of quartz. Traced 
towards the granite, they present an increasingly pronounced meta- 
morphism. First they assume a spotted appearance, owing to the 
development of small dark points and knots, which increase in size and 
number towards the granite, while the ground-mass remains unaltered 
(Knotenschiefer, Fruchtschiefer). The ground-mass of the slate then 
becomes lighter in colour, harder, and more crystalline in appearance, 
while flakes of mica and quartz-grains make their appearance. The 
knots, now broken up, rather increase than diminish in size; the 
hardness of the rock rapidly increases, and the fissile structure becomes 
unrecognizable on a fresh fracture, though observable on a weathered 
surface. Still nearer the granite, the knot-like concretions disappear 
from the rock, which then has become an entirely crystalline mass, in 
which, with the lens, small flakes of mica and grains of quartz can be 
seen, and which under the microscope appears as a thoroughly crystalline 
aggregate of andalusite, quartz, and mica. The proportions of the 
ingredients vary, but the andalusite and quartz usually greatly pre- 
ponderate (andalusite-schist). Chemical analysis shows that the un- 
altered clay-slate and the crystalline andalusite-schist next the granite 
consist essentially of similar chemical materials, and that “probably the 
metamorphism has not taken place by the addition or subtraction of 
matter, but by another and still unknown process of molecular trans- 
position.”* In some cases boric acid has been supplied to the schists at 
the contact.® 
J. Horne, Mem. Geol. Survey, Scotland, Explanation of Sheet 9, p. 22. The fine 
“pneiss” found as a contact product round the granite of Devon and Cornwall was 
termed “cornubianite” by Boase—a name which Naumann has proposed to devote to this 
kind of rock. (Geol. i. 548. 
* N. Jahrb. 1870, p. 742. 
8 Op. cit. 1875, p. 849. “Die Steigerschiefer und ihr act- ” Str 
1877, ‘Unger, N. Jahrb. 1876, p. 785. ° Contict-Zone a 
* Unger, Op. cit. p. 806. 
* Rosenbusch, “ Die Steigerschiefer,” &c.,, p. 257. 
