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NATURE 
413 
It will be observed, in regard to these theories, that none of 
them supposes that the old gneiss is an ordinary sediment, but 
that all regard it as formed in exceptional circumstances, these 
circumstances being the absence of land and of sub-aerial decay 
of rock, and the presence wholly or principally of the material 
of the upper surface of the recently hardened crust. This being 
granted, the question arises, Ought we not to combine these 
several theories and to believe that the cooling crust has hardened 
in successive layers from without inward ; that at the same time 
fissures were locally discharging igneous matter to the surface ; 
that matter held in suspension in the ocean, and matter held in 
solution by heated waters rising from beneath the outer crust 
were mingling their materials in the deposits of the primitive 
ocean? Jt would seem that the combination ofall these agencies 
may safely be invoked as causes of the pre-Atlantic deposits. 
This is the eclectic position which I endeavoured to maintain in 
my address before the Minneapolis meeting of the American 
Association in 1883, and which I still hold to be in every way 
probable. 
A word here as to metamorphism, a theory which, like many 
others, has been first run to death and then discredited, but 
which to the moderate degree in which it was originally held by 
Lyell is still valid. Nothing can be more certain than that the 
composition of the Laurentian gneisses forbids us to suppose that 
they can be ordinary sediments metamorphosed. They are 
rocks peculiar in their origin, and not paralleled unless excep- 
tionally in later times. On the other hand, they have un- 
doubtedly experienced very important changes, more especially 
as to crystallisation, the state of combination of their ingredients, 
and the development of disseminated minerals ;! and while this 
may in part be attributed to the mechanical pressure to which 
they have been subjected, it requires also the action of hydro- 
thermic agencies. Any theory which fails to invoke both of 
these kinds of force must necessarily be partial and imperfect. 
But all metamorphic rocks are not of the same character with 
the gneisses of the Lower Laurentian. Even in the Middle 
and Upper Laurentian we have metamorphic rocks, e.g. quartzite 
and limestone, which must originally have been ordinary aqueous 
deposits. Still more in the succeeding Huronian and its asso- 
ciated series of beds, and in the Lower Paleozoic, local meta- 
morphic change has been undergone by rocks quite similar to 
those which in their unaltered state constitute regular sedimentary 
deposits. In the case of these later rocks it is to be borne in 
mind that, while some may have been of volcanic origin, others 
may have been sediments rich in undecomposed fragments of 
silicates. It isa mistake to suppose that the ordinary decay of 
stratified siliceous rocks is a process of kaolinisation so perfect as 
to eliminate all alkaline matters. On the contrary, the fact, 
which Judd has recently well illustrated in the case of the mud 
of the Nile, applies to a great number of similar deposits in all 
parts of the world, and shows that the finest sediments have not 
usually been so completely lixiviated as to be destitute of the 
basic matters necessary for their conversion into gneiss, mica- 
schist, and similar rocks when the necessary agencies of meta- 
morphism are applied to them, and this quite independently of 
any extraneous matters introduced into them by water or other- 
wise. Still it must be steadily kept in view that many of the 
old pre-Cambrian crystalline rocks must have been different 
originally from those succeeding them, and that consequently 
these last even when metamorphosed present different characters, 
I may remark here that, though a paleontologist rather than 
a lithologist, it gives me great pleasure to find so much attention 
now given in this country to the old crystalline rocks, and to 
their study microscopically and chemically as well as in the 
field, a work in which Sorby and Allport were pioneers. Asa 
pupil of the late Prof. Jameson, of Edinburgh, my own attention 
was early attracted to the study of minerals and rocks as the 
stable foundations of geological science ; and as far back as 1841 
I had learnt of the late Mr. Sanderson, of Edinburgh, who 
worked at Nicol’s sections, how to slice rocks and fossils ; and 
since that time I have been in the habit of examining everything 
and doleritic magmas, which have been conjectured to be the sources of two 
great types of eruptive rocks. Inasmuch, however, as according to the 
present hypothesis these two layers of basic and acidic matters are the 
results of aqueous action, and not of an original separation in a plutonic mass, 
as imagined by Phillips and Durocher, their composition would be subiect to 
many local variations.” ; 
* The first of these is what Bonney has called Metastasis. The second 
and third come under the name Metacrasis. Methylosrs, or change of 
substance, is altogether exceptional, and not to be credited except on the 
best evidence, or in cases where volatile matters have been expelled, as in 
the change of hematite into magnetite, or of bituminous coalinto anthracite. 
with the microscope. The modern developments in this direction 
are therefore very gratifying, even though, as is natural, they 
may sometimes appear to be pushed to» far or their value 
over-estimated. 
That these old gneisses were deposited not only in what is 
now the bed of the Atlantic, but also on the great continental 
areas of America and Europe, any one who considers the wide 
extent of these rocks represented on the map recently published 
by Prof. Hull can readily understand (7yavzs. Royal Irish 
Academy). It is true that Hull supposes that the basin of the 
Atlantic itself may have been land at this time, but there is no 
evidence of this, more especially as the material of the gneiss 
could not have been detritus derived from sub-aérial decay of 
rock. 
Let us suppose, then, the floor of old Ocean covered with a 
flat pavement of gneiss, or of that material which is now gneiss, 
the next question is, How and when did this original bed become 
converted into seaand land? Here we have some things certain, 
others most debatable. That the cooling mass, especially if it 
was sending out volumes of softened rocky material, either in 
the exoplutonic or in the crenitic way, and piling this on the 
surface, must soon become too small for its shell, is apparent ; 
but when and where would the collapse, crushing, and wrinkling 
inevitable from this cause begin? Where they did begin is 
indicated by the lines of mountain-chains which traverse the 
Laurentian districts ; but the reason why is less apparent. The 
more or less unequal cooling, hardening, and conductive power 
of the outer crust we may readily assume. The driftage un- 
equally of water-borne detritus to the south-west by the bottom 
currents of the sea is another cause, and, as we shall soon see, 
most effective. Still another is the greater cooling and harden- 
ing of the crust in the polar regions, and the tendency to collapse 
of the equatorial protuberance from the slackening of the earth’s 
rotation. Besides these the internal tides of the earth’s substance 
at the times of solstice would exert an oblique pulling force on 
the crust, which might tend to crack it along diagonal lines. 
From whichever of these causes or the combination of the whole, 
we know that within the Laurentian time folded portions of the 
earth’s crust began to rise above the general surface in broad 
belts running from north-east to south-west, and from north-west 
to south-east, where the older mountains of Eastern America 
and Western Europe now stand, and that the subsidence of the 
oceanic areas allowed by this crumpling of the crust permitted 
other areas on both sides of what is now the Atlantic to form 
limited table-lands.!_ This was the beginning of a process re- 
peated again and again in subsequent times, and which began 
in the Middle Laurentian, when for the first time we find beds 
of quartzite, limestone, and iron ore, and graphitic beds, in- 
dicating that there was already land and water, and that the sea, 
and perhaps the land, swarmed with animal and plant life of 
forms unknown to us for the most part now. Independently of 
the questions as to the animal nature of Eozoon, I hold that we 
know, as certainly as we can know anything inferentially, of the 
existence of these primitive forms of life. If I were to conjecture 
what were the early forms of plant and animal life, I would 
suppose that just as in the Palaeozoic the acrogens culminated in 
gigantic and complex forest trees, so in the Laurentian the Algze, 
the lichens, and the mosses grew to dimensions and assumed 
complexity of structure unexampled in later times, and that in 
the sea the humbler forms of Protozoa and Hydrozoa were the 
dominant types, but in gigantic and complex forms. The land 
of this period was probably limited, for the most part, to high 
latitudes, and its aspect, though more rugged and abrupt, and 
of greater elevation, must have been of that character which we 
still see in the Laurentian hills. The distribution of this ancient 
land is indicated by the long lines of old Laurentian rock extend- 
ing from the Labrador coast and the north shore of the St. 
Lawrence, and along the eastern slopes of the Appalachians in 
America, and the like rocks of the Hebrides, the Western High- 
lands, and the Scandinavian mountains. A small but interesting 
remnant is that in the Malvern Hills, so well described by Holl. 
Ir will be well to note here, and to fix on our minds, that these 
ancient ridges of Eastern America and Western Europe have 
been greatly denuded and wasted since Laurentian times, and 
that it is along their eastern sides that the greatest sedimentary 
accumulations have been deposited. 
From this time dates the introduction of that dominance of 
t Daubrée’s curious experiments on the contraction of caoutchouc balloons 
partially hardened by coating with varnish, shows how small inequalities of 
the crust, from whatever cause arising, might affect the formation of wrinkles, 
and also that transverse as well as Jongitudinal wankling might occur, 
