412 
may also be carried to the surface by the agency of heated 
water, producing those quiet discharges which Hunt has named 
crenitic. It is to be observed here that explosive voleanic phe- 
nomena, and the formation of cones, are, as Prestwich has well 
remarked, characteristic of an old and thickened crust; quiet 
ejection from fissures and hydro-thermal action may have been 
more common in earlier periods and with a thinner over-crust. 
(6) The contraction of the earth’s interior by cooling and by 
the emission of material from below the over-crust, has caused 
this crust to press downward, and therefore laterally, and so to 
effect great bends, folds, and plications; and these modified 
subsequently by surface denudation constitute mountain-cbains 
and continental plateaus. As Hall long ago pointed out,! such 
lines of folding have been produced more especially where thick 
sediments had been laid downon the sea-bottom. Thus we have 
here another apparent paradox, namely, that the elevations of the 
earth’s crust occur in the places where the greatest burden of 
detritus has been laid down upon it, and where consequently the 
crust has been softened and depressed. We must beware, in 
this connection, of exaggerated notions of the extent of contrac- 
tion and of crumbling required to form mountains. Bonney has 
well shown, in lectures delivered at the London Institution, that 
an amount of contraction, almost inappreciable in comparison 
with the diameter of the earth, would be sufficient ; and that as 
the greatest mountain-chains are less than 1/600 of the earth’s 
radius in height, they would on an artificial globe a foot in dia- 
meter be no more :mportant than the slight inequalities that 
might result from the paper gores overlapping each other at the 
edges. 
(7) The crushing and sliding of the over-crust inplied in these 
movements raise some serious questions of a physical character. 
One of these relates to the rapidity or slowness of such move- 
ments, and the consequent degree of intensity of the heat 
developed, as a possible cause of metamorphism of rocks. 
Another has reference to the possibility of changes in the equili- 
brium of the earth itself as resulting from local collapse and 
riaging, These questions in connection with the present disso- 
ciation of the axis of rotation from the magnetic poles, and with 
changes of climate, have attracted some attention,” and probably 
deserve further consideration on the part of physicists. In so 
far as geological evidence is concerned, it would seem that the 
general association of crumbling with metamorphism indicates a 
certain rapidity in the process of mountain-making, and conse- 
quent development of heat, and the arrangement of the older 
rocks around the Arctic basin forbids us from assuming any 
extensive movement of the axis of rotation, though it does not 
exclude changes to a limited extent. I hope that Prof. Darwin 
will discuss these points in his address to the Physical Section. 
I wish to formulate these principles as distinctly as possible, 
and as the result of all the long series of observations, calcula- 
tions, and discussions since the time of Werner and Hutton, and 
in which a vast number of able physicists and naturalists have 
borne a part, because they may be considered as certain deduc- 
tions from our actual knowledge, and because they lie at the 
foundation of a rational physical geology. 
We may popularise these deductions by comparing the earth 
to a drupe or stone-fruit, such as a plum or peach, somewhat 
dried up. It has a large and intensely hard stone and kernel, a 
thin pulp made up of two layers, an inner more dense and dark- 
coloured, and an outer less dense and lighter-coloured. These 
constitute the under-crust. On the outside it has a thin membrane 
or over-crust. In the process of drying it has slightly shrunk, 
so as to produce ridges and hollows of the outer crust, and this 
outer crust has cracked in some places, allowing portions of the 
pulp to ooze out—in some of these its lower dark substance, in 
others its upper and lighter material. The analogy extends no 
further, for there is nothing in our withered fruit to represent 
the oceans occupying the lower parts of the surface or the deposits 
which they have laid down. 
Keeping in view these general conclusions, let us now turn to 
their bearing on the origin and history of the North Atlantic. 
Though the Atlantic is a deep ocean, its basin does not con- 
stitute so much a depression of the crust of the earth as a 
flattening of it, and this, as recent soundings have shown, with a 
slight ridge or elevation along its middle, and banks or terraces 
t Hall (American Association Address, 1857, subsequently republished, with 
additions, as ‘‘ Contributions to the Geological History of the American Conti- 
nent,’’ Mallet), Rogers, Dana, Le Conte, &c, 
* See recent papers of Oldham and Fisher in the Geological Magazine and 
Philosophical Magazine, July 1886. Also Péroche, *‘ Revol. Polaires’’ 
(Paris, 1886). 
NATURE 
[Sep7. 2, 1886 
fringing the edges, so that its form is not so much that of a basin 
as that of a shallow plate with its middle alittle raised. Its true 
permanent margins are composed of portions of the over-crust 
folded, ridged up, and crushed, as if by lateral pressure emanat- 
ing from the sea itself. We cannot, for example, look at a 
geological map of America without perceiving that the Appala- 
chian ridges, which intervene between the Atlantic and the St. 
Lawrence Valley, have been driven bodily back by a force acting 
from the east, and that they have resisted this pressure only 
where, as in the Gulf of St. Lawrence and the Catskill region 
of New York, they have been protected by outlying masses of 
very old rocks, as, for example, by that of the Island of New- 
foundland and that of the Adirondack Mountains. The admir- 
able work begun by my friend and fellow-student,. Prof. James 
Nicol, followed up by Hicks, Lapworth, and others, and now, 
after long controversy, fully confirmed by the recent observations 
of the Geological Survey of Scotland, has shown the most intense 
action of the same kind on the east side of the ocean in the 
Scottish highlands ; and the more widely distributed Eozoic 
rocks of Scandinavia may be appealed to in further evidence of 
this. 
If we now inquire as to the cause of the Atlantic depression, 
we must go back to a time when the areas occupied by the 
Atlantic and its bounding coasts were parts of a shoreless sea in 
which the earliest gneisses or stratified granites of the Laurentian 
age were being laid down in vastly extended beds. These 
ancient crystalline rocks have been the subject of much discus- 
sion and controversy, and as they constitute the lowest and 
probably the firmest part of the Atlantic sea-bed, it is necessary 
to inquire as to their origin and history. Dr. Bonney, the late 
President of the Geological Society, in his anniversary address, 
and Dr. Sterry Hunt, in an elaborate paper communicated to 
the Royal Society of Canada, have ably summed up the hypo- 
theses as to the origin of the oldest Laurentian beds. At the 
thick beds of orthoclase gneiss, which are the oldest stratified 
rocks known to us, are substantially the same in composition 
with the upper or siliceous magma or layer of the under-crust. 
They are, in short, its materials either in their primitive condi- 
tion or merely re-arranged. One theory considers them as 
original products of cooling, owing their lamination merely to 
the successive stages of the process. Another view refers them 
to the waste and re-arrangement of the materials of a previously 
massive granite. Still another holds that all our granites really 
arise from the fusion of old gneisses of originally aqueous origin ; 
while a fourth refers the gneisses themselves to molecular 
changes effected in granite by pressure. These several views, 
in so far as they relate to the oldest or fundamental Laurentian 
gneiss, may be arranged under the following heads :—(1) Exdo- 
plutonic, or that which regards all the old gneisses as molten 
rocks cooled from without inward in successive layers.? (2) 
Lxoplutonic, or that which considers them as made up of matter 
ejected from below the upper crust in the manner of volcanic 
action.’ (3) Metamorphic, which supposes the old gneisses to 
arise from the crystallisation of detrital matter spread over the 
sea-bottom, and either igneous or derived from the decay of 
igneous rocks,4 (4) Chaotic or Thermo-chaotic, or the theory of 
deposit from the turbid waters of a primeval ocean either with 
or without the aid of heat.® In one form this was the old theory 
of Werner. (5) Crenitic or Hydro-thermic, which supposes the 
action of heated waters penetrating below the crust to be con- 
stantly bringing up to the surface mineral matters in solution and 
depositing these so as to form feldspathic and other rocks.® 
* Address to the Geological Section, by Prof. Judd, Aberdeen meeting, 
1885. According to Rogers, the crumpling of the Appalachians has reduced 
a breadth of 158 miles to about 60. 
2 Naumann, Phillips, Durocher, McFarlane, &c. 
3 Clarence King, Tornebohm, Marr, &c. 
4 Lyell, Kopp, Reusch, Judd, &c. 5 Scrope, Delabeche, Daubrée, _ 
6 Hunt, doc. cit. The following is Dr. Hunt’s summary statement of this 
theory :—*‘ The globe consolidating at the centre left, it is conceived, a super- 
ficial layer of basic silicates, which has yielded all the fixed elements of the 
earth’s crust. 
sea, the acid waters of which, permeating and partially decomposing it, 
became thereby chemically neutralised. This last-cooled layer, mechanically 
disintegrated, saturated with water, and heated by the central mass, was the 
source of mineral springs, holding in solution the silicates which built up the 
ancient gneisses and similar rocks. Granitic veins and zeolites are due to 
survivals of the process which generated the gneissic rocks. “The hypothesis 
of their formation from materials brought to the surface by mineral springs 
from the primitive basic layer affords, it is claimed, the elements ofa com- 
plete and intelligible explanation of the origin of the Eozoic rocks. ‘Ihis 
upward lixiviation of the primitive mass, and the deposition over it of an 
acidic granite-like rock, would leave below a highly basic material, aad the 
, division of the mass thus established would correspond to that of the trachytic 
basis of these hypotheses lies the admission that the immensely _ 
This layer formed the first land and the floor of the primeval’ 
0 thy ee mg 
