Sept. 17, 1885 | 
NATURE 
475 
about by earlier movements would of course exercise a certain 
though subordinate influence in producing the existing forms of 
the surface of the country. 
At the close of the Jurassic period, and before the commence- 
ment of the Cretaceous, during the vast epoch marked by the 
deposition of the Neocomian of Southern Europe, a series of 
disturbances similar to those of the Tertiary, and scarcely 
inferior in their consequences, can be shown to have taken 
place. 
If the movements of the Scandinavian and Scottish rock- 
masses which took place in the Tertiary and Mesozoic periods 
respectively were so startling in their magnitude and so vast in 
their effects, what shall we say concerning those far greater dis- 
turbances which affected the same area towards the close of the 
Older Paleozoic and the beginning of the Newer Palzeozoic, 
when this Northern Alps was still a living and growing mountain- 
chain ? 
These movements, in which both the Archzan and the Older- 
Palzozcic rocks are found to be involved, have resulted in the 
production, through enormous lateral pressure, of those reversed 
faults, caused by the disruption along their axial planes of 
greatly inclined and compressed folds, as so well described by 
Rogers. 
Dr. Archibald Geikie assures us that the studies of the 
geological surveyors in North-West Sutherland led to the con- 
clusion that certain masses of rock have thus been carried almost 
horizontally over others, along these ‘‘ thrust-planes” for a dis- 
tance of at least ten miles. As the result of these tremendous 
lateral compressions, thin beds of limestone and quartzite, which 
have sufficiently definite characters to permit of their recognition, 
may be seen in Assynt, and in other parts of the Western High- 
lands, to be so repeated again and again by crumpling and fault- 
ing, that they have been regarded as deposits of enormous 
thickness ; while, on the other hand, massive formations have 
been crushed and rolled out, thereby acquiring a laminated 
structure like so much pie-crust. Great portions of rock-masses, 
which, like the much-discussed ‘* Logan-rock,” have been nipped 
between gigantic faults, show evidence under the microscope of 
having been crushed to powder and subsequently reconsolidated, 
while the surfaces of the ‘‘thrust-planes”’ sometimes exhibit 
the phenomena known as ‘‘slickensides” on the most gigantic 
scale. 
As we pass away from the central axis of this old mountain- 
chain, however, these complicated puckerings and dislocations 
pass gradually into more ordinary folds and faults, just as is the 
case with the Appalachians. The oft-repeated undulations of 
the Lower Paleozoic strata of the Borderland, so admirably 
described by Professor Lapworth, bear the same relation to the 
far more involved disturbances of rocks of the same age in the 
Highlands, which the foldings of the strata in the Jura do to the 
intense crumplings of those of the Alps; and these in turn pass 
insensibly into the slightly undulating or horizontal strata of the 
-southern half of this island. 
We may perhaps add another comparison between the existing 
mountain-chain of Southern Europe and the ‘ basal wreck ” of 
Northern Europe, one which I find has been already suggested 
by Professor Bonney. The Miocene Conglomerates, which in 
the Rigi and other flanking mountain masses of the Alpine 
chain are found piled to the depth of many thousands of feet, 
seem to be exactly represented in its prototype by the vast masses 
of the ‘* Old-Red ” Conglomerate. 
Vast as were the three series of movements to which I have 
been referring, I believe that the Scandinavian and Highland 
rocks bear the impress of a still grander series of di turbances 
than either of these—one at the same time of older date and far 
more universal in its effects. 
Many writers have treated of the great divisional planes, al- 
most everywhere conspicuous in the Highland rock-masses, as 
being necessarily coincident with planes of sedimentation. It is 
manifest, indeed, that the tracing of sequences and unconformi- 
ties among such rocks must proceed upon the assumption that 
the planes of foliation and stratification ave coincident. Murchi- 
son and Geikie so fully recognised the fact that this proposition 
lay at the very root of their arguments concerning a Highland 
succession, that they added a supplement to their paper to 
illustrate and enforce it. 
It must not be forgotten, however, that the truth of this pro- 
position has not only been doubted, but has heen stoutly con- 
tested by many of the most profound thinkers on geological 
questions, 
q 
As long ago as 1822, Professor Henslow, ina very remarkable 
paper, showed that the rocks of Anglesea are traversed by a 
system of divisional planes, which intersect the bedding at a very 
high angle, and must have been produced long subsequently to 
the latter ; and in 1835 Professor Sedgwick extended the obser- 
vations and enforced the arguments of Henslow. 
At an even earlier date, Poulett Scrope had shown, by his 
study of viscous lavas, that the planes along which crystalline 
action takes place are determined by pressure and strain ; and 
he insisted that the foliation of metamorphic masses was a 
phenomenon strictly analogous to the banding of rhyolitic 
lavas. 
Charles Darwin, the pupil of Henslow and the friend of 
Poulett Scrope—whose labours in the geological field would 
perhaps have met with fuller recognition had they not been 
overshadowed by his still greater achievements in the world of 
biological thought—strongly maintained the truth of these views. 
He added the important observation that, in the South 
American continent, the planes of foliation are seen everywhere, 
over enormous areas, to be parallel to those of cleavage ; and 
that these latter are of secondary origin and due to lateral pres- 
sure, the observations of Sharpe and the experiments of Sorby 
have convincingly demonstrated. 
That the schists and gneisses of our Highlands and of Scand- 
inavia have resulted from crystallising forces, acting upon strata 
of sandstone, clay, and limestone, or upon igneous materials 
constituting lava-currents, or intrusive sheets, dykes, and bosses,. 
I see every reason for believing. That these re-crystallised and 
highly-foliated masses in the great majority of cases maintain 
their original positions and relations, or indeed anything ap- 
proaching their original positions and relations, I greatly doubt ; 
and my doubt on this point has increased the more I have studied 
the Highland rocks. 
Thin bands of quartzite may be the rolled-out representatives 
of massive beds of sandstone or conglomerate ; wide-spreading 
schists may consist of the crystallised materials of clays and 
shales, crumpled, pleated, and kneaded together in endless con- 
volutions ; vast sheets of gneiss may have originally been in- 
trusive bosses of granite or thick strata of arkose. How, then, 
are we to apply the ordinary principles that regulate questions 
concerning dip and strike, and unconformity in the case of 
sedimentary deposits, to highly altered rocks like these ? 
The observations of Jukes, Allport, and Phillips on some of 
the simpler and more easily explicable examples of the produc- 
tion of foliation in rocks require to be cautiously extended, by 
patient study in the field and in the laboratory, to cases of a 
more complex and difficult character. Especially in this con- 
nection do we welcome such contributions to our knowledge as 
that made by Mr. Teall in his description of the remarkable 
foliated dyke of Scourie. 
Very significant indeed is the fact that the phenomena of 
foliation appears to be confined to regions which have been the 
scene of the most violent subterranean movement and disturb- 
ance. That solid rock-masses, subjected to the tremendous 
earth-strains to which they are liable during mountain-making, 
are capable of internal movement and flow—like the ice of a 
glacier—we have the clearest evidence. Many illustrations 
might be adduced in support of the view that crystallisation is 
influenced and controlled by mechanical forces—pressures, 
stresses, and strains. May it not also be true, as long ago sug- 
gested by Vose, that the heat which must be generated in the 
great shearing movements taking place in rocks have also had 
much to do in giving rise to that re-crystallisation which is the 
essence of foliation? Rock-masses, in the throes of mountain- 
birth, have, like glaciers, behaved substantially as viscous bodies ; 
may not the former have undergone molecular changes analogous 
to regelation in the latter? 
That many of the stupendous earth-movyements which produced 
the foliation of the rocks of Scandinavia and the Scottish High- 
lands must be referred to Archzean times, there is not the smallest 
room for doubt. That similar effects have resulted from the 
same agencies during subsequent periods, our fellow-geologists 
in Scandinavia believe they have found incontrovertible proof. 
For my own part I look forward confidently to the establish- 
ment of the same conclusion from the study of our own Highland 
rocks. “ 
But here I am conscious that I am venturing on topics upon 
which great and allowable differences of opinion still exist. 
The debates in this Geological Section during the first meeting 
of the British Association in Aberdeen ought, I think, to have 
