552 REPORT—1904. 
two or more periods of infiltration, followed by movements which have broken 
up the previously deposited material, consisting of quartz, tourmaline, and tin- 
stone. 
In the cases above referred to the evidence of the mechanical fracture of the 
rocks and of their constituent minerals is obvious, but in those which have now to 
be considered such evidence is rare or altogether absent. 
The Lewisian gneiss of Scotland is traversed by alarge number of basic dykes. 
Both gneiss and dykes are crossed by shear-zones in which the rocks have been 
deformed and in which the structure and, to some extent, the mineralogical com- 
position of the original rocks have been changed. The basic dykes have become 
hornblende schists and the gneiss a hornblende granulite. Both rocks are now in 
the condition of holocrystalline schists, and although traces of mechanical fracture 
may sometimes be seen in the transitional forms, they are as a rule conspicuous by 
their absence, and are not found in the finished product. 
The essential difference between fault-breccias and mylonites, on the one hand, 
and the holocrystalline schists of the shear-zones on the other, is that evidence of 
mechanical fracture, both of the rocks and of their mineral constituents, is 
abundant in the former and rare or absent in the latter. This difference is pro- 
bably due to the fact that the deformation in the case of the shear-zones took 
place under a greater load and at a higher temperature. 
The breadth of many of the typical shear-zones is only a few yards, but wider 
belts of country composed of similar rocks—hornblende schists and granulitic 
gaeisses—occur, so that the consideration of these shear-zones helps to bridge over 
the gap between the two classes of effects—the local and the regional. 
Regional effects are well illustrated by the phenomena of slaty cleavage, which 
is due to the mechanical deformation of extensive tracts of country. Sharpe and 
Darwin associated the foliation of crystalline schists with slaty cleavage, an 
association which appears to be justifiable, although the case as stated by them 
requires some modification. Much remains to be done before the problem of the 
origin of the crystalline schists is solyed, but a few points of considerable import- 
ance have been definitely established. 
Taking the Highlands of Scotland as an example, the foliated crystalline rocks 
of that region are, as a rule, easily separable into two distinct classes—those of 
igneous and those of sedimentary origin. 
In dealing with foliated igneous rocks there is, in many cases, a doubt as to 
whether the foliation may not date from the time of intrusion and be of the nature 
of original fluxion; but when a granitic mass, its apophyses and the metamorphosed 
sediments on its margin have a common foliation, and the apophyses are foliated 
transversely to their width, and not parallel to their margins, we are compelled to 
assume that the foliation has been produced by earth movements operating 
after intrusion, consolidation, and contact-metamorphism. This is the case, for 
example, with the Carn Chivinneag mass in Eastern Ross, which has recently 
been investigated by Mr, Clough. 
Crystalline schists of sedimentary origin also form large tracts in the Highlands. 
They probably belong to more than one formation, and certainly include repre- 
sentatives of arenaceous, argillaceous, and calcareous types. To what extent the 
present condition of these crystalline schists is to be attributed to earth movements 
is more or less an open question. That they have been powerfully affected by such 
movements is often clearly proved by their disposition, by the presence of recog- 
nisable folds, and by the flattening or elongation of the clastic grains and pebbles 
in the coarser-grained sediments. If foliation be taken to include both parallel 
banding and the disposition of minerals with their longer axes in one definite 
direction, it is probable that in general the former is evidence of original stratifi- 
cation, although the thickness of a band, as we now see it, may be different from 
its original thickness, and the latter is the result of earth movement. 
It appears, therefore, that the secondary foliation of igneous rocks and a part 
! There is also a third class of mixed rocks, due to the impregnation of sedi 
mentary with igneous material, but this class is relatively unimportant. 
