344 Messrs. Clough, Crampton, and Flett—Augen Gneiss. 
Still more convincing evidence, if that were possible, that the 
schists and gneisses of this country were developed from solid rocks 
by pressure and movement is furnished by a series of remarkable 
crush-zones that traverse both igneous and sedimentary rocks alike. 
Some of these were described by Mr. Clough in 1902,’ but the main 
facts may be recapitulated here. 
After the movements that produced the main foliation of the 
district had come to an end, and the rocks were for the most part in 
the same state as they are at present, a group of basic dykes (olivine 
dolerites) were injected along a system of fissures that have mainly 
a N.N.W. trend. Thereafter movements again set in (posthumous 
movements), but on a lesser scale and more local in their distribution. 
They folded the previous foliation in the gneisses and schists, and 
their action appears in a concentrated form along certain crush-zones 
or belts of secondary shearing, which are narrow strips running in 
various directions, but often nearly at right angles to the strike of the 
general folding in the Moine rocks. Where the hornfels is involved 
in one of these crush-zones it may be changed within a few feet into 
a lustrous mica schist. When a crush-zone passes from the schists 
into the granite gneiss, the latter also becomes highly schistose with 
a large development of new white mica. In the granite gneiss the 
vertical dolerite dykes were apparently weak rocks that easily yielded 
to pressure, for secondary shear zones rather commonly take the line 
of these dykes. In that case the basic rock assumes the form of 
a typical hornblende schist, rather fine-grained but perfectly foliated 
and completely metamorphic. The foliation in the crush-zones is 
parallel to the length of the zone, and consequently is often at right 
angles to the foliation of the Moine rocks and the granite gneiss. At 
the edges of the hornblende schist dykes a new foliation makes its 
appearance in the granite gneiss. Most perfect at the junction with 
the dyke, it fades away in a few inches. It is evidently a superinduced 
structure which masks the original foliation of the acid rock, yet it is 
often very highly developed, and thin slabs may be split off from the 
eneiss at the edge of the basic dykes with a structure which may be 
described as mylonitic at a considerable angle to the foliation in the 
same rock only a couple of feet away. 
The fine-grained structure of the dykes proves the granite to have 
been cold at the time when they were injected, and the development 
of a new foliation in the gneiss that forms the walls of the dykes proves 
equally clearly that the main foliation was already developed before 
these rocks were sheared. When the later movements supervened the 
granite compressed the dykes as a vice grips a piece of soft iron. The 
weaker metal gives way and is deformed, but before its rigidity is 
overcome the jaws of the vice also suffer. 
We believe that the granite gneiss and its aureole furnish us with 
an undoubted example of pure dynamic metamorphism. All the rocks 
involved—igneous, sedimentary, and the contact-altered hornfelses— 
were free from foliation when the movements began, and when the 
movements came to an end they were, in varying degrees of perfection, 
1 Summary of Progress of the Geological Survey for 1902, p. 150. 
a eee 
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