Sept. 2, 1886] 
NATURE 
411 
evidence of the general views announced, I trust you will kindly 
attribute this to the exigencies of a short address. 
If we imagine an observer contemplating the earth from a 
convenient distance in space, and scrutinising its features as it 
rolls before him; we may suppose him to be struck with the fact 
that eleven-sixteenths of its surface are covered with water, and 
that the land is so unequally distributed that from one 
point of view he would see a hemisphere almost exclu- 
sively oceanic, while nearly the whole of the dry land is 
gathered in the opposite hemisphere. He might observe that 
the great oceanic area of the Pacific and Antarctic Oceans is 
dotted with islands—like a shallow pool with stones rising above 
its surface—as if its general depth were small in comparison 
with its area. He might also notice that a mass or belt of land 
surrounds each Pole, and that the nerthern ring sends off to the 
southward three vast tongues of Jand and of mountain-chains, 
terminating respectively in South America, South Africa, and 
Australia, towards which feebler and insular processes are given 
off by the Antarctic continental mass. This, as some geo- 
graphers have observed,! gives a rudely three-ribbed aspect to 
the earth, though two of the three ribs are crowded together and 
form the Europ-asian mass, or double continent, while the third 
is isolated in the single continent of America. He might also 
observe that the northern girdle is cut across, so that the 
Atlantic opens by a wide space into the Arctic Sea, while the 
Pacific is contracted towards the north, but confluent with the 
Antarctic Ocean. The Atlantic is also relatively deeper and 
less cumbered with islands than the Pacific, which has the 
higher ridges near its shores, constituting what some visitors to 
the Pacific coast of America have not inaptly called the ‘‘ back 
of the world,” while the wider slopes face the narrower ocean, 
into which for this reason the greater part of the drainage of the 
land is poured.* The Pacific and Atlantic, though both depres- 
sions or flattenings of the earth, are, as we shall find, different 
in age, character, and conditions ; and the Atlantic, though the 
smaller, is the older, and, from the geological point of view, in 
some respects the more important of the two. 
[f our imaginary observer had the means of knowing anything 
of the rock-formations of the continents, he would notice that 
those bounding the North Atlantic are in general of great age, 
some belonging to the Laurentian system. On the other hand, 
he would see that many of the mountain-ranges along the Pacific 
are comparatively new, and that modern igneous action occurs 
in connection with them. Thus he might be led to believe that 
the Atlantic, though comparatively narrow, is an older feature 
of the earth’s surface, while the Pacific belongs to more modern 
times. But he would note in connection with this that the 
oldest rocks of the great continental masses are mostly toward 
their northern ends, and that the borders of the northern ring of 
land and certain ridges extending southwards from it constitute 
the most ancient and permanent elevations of the earth’s crust, 
though now greatly surpassed by mountains of more recent age 
nearer the equator. Before leaving this general survey we may 
make one further remark. An observer looking at the earth 
from without would notice that the margins of the Atlantic and 
the main lines of direction of its mountain-chains are north-east 
and south-west, and north-west and south-east, as if some early 
causes had determined the occurrence of elevations along great 
circles of the earth’s surface tangent to the polar circles. 
We are invited by the preceding general glance at the surface 
of the earth to ask certain questions respecting the Atlantic :— 
(1) What has at first determined its position and form? (2) 
What changes has it experienced in the lapse of geological time ? 
(3) What relations have these changes borne to the development 
of life on the Jand and in the water? (4) What is its probable 
future ? 
Before attempting to answer these questions, which I shall 
not take up formally in succession, but rather in connection 
with each other, it is necessary to state as briefly as possible 
certain general conclusions respecting the interior of the earth. It 
is popularly supposed that we know nothing of this beyond a super- 
ficial crust perhaps averaging 50,000 to 100,000 feet in thickness. 
It is true we have no means of exploration in the earth’s interior, 
but the conjoined labours of physicists and geologists have now 
proceeded sufficiently far to throw much inferential light on the 
subject, and to enable us to make some general affirmations with 
* Dana, ‘‘ Manual of Geology,” introductory part. Green, ‘‘ Vestiges of a 
Molten Globe,’’ has summed up these facts. 
* Mr. Mellard Reade, in two Presidential addresses before the Geological 
Society of Liverpool, has well illustrated this point and its geological con- 
sequence. 
certainty ; and these it is the more necessary to state distinctly, 
since they are often treated as mere subjects of speculation and 
fruitless discussion. 
(1) Since the dawn of geological science it has been evident 
that the crust on which we live must be supported on a plastic 
or partially liquid mass of heated rock, approximately uniform 
in quality under the whole of its area. This is a legitimate con- 
clusion from the wide distribution of voleanic phenomena, and 
from the fact that the ejections of volcanoes, while locally of 
various kinds, are similar in every part of the world. It led to 
the old idea of a fluid interior of the earth, but this is now gene- 
rally abandoned, and this interior heated and plastic layer is 
regarded as merely an under-crust. f 
(2) We have reason to believe, as the result of astronomical 
investigations,! that, notwithstanding the plasticity or liquidity 
of the under-crust, the mass of the earth—its nucleus as we may 
call it—is practically solid and of great density and hardness. 
Thus we have the apparent paradox of a solid yet fluid earth, 
solid in its astronomical relations, liquid or plastic for the pur- 
poses of volcanic action and superficial movements.” 
(3) The plastic sub-crust is not in a state of dry igneous 
fusion, but in that condition of aqueo-igneous or hydro-thermic 
fusion which arises from the action of heat on moist substances, 
and which may either be regarded as a fusion or as a species of 
solution at a very high temperature. This we learn from the 
phenomena of volcanic action, and from the composition of the 
volcanic and plutonic rccks, as well as from such chemical 
experiments as thosp of Daubrée and of Tilden and Shenstone.* 
(4) The interior sub-crust is not perfectly homogeneous, Lut 
may be roughly divided into two layers or magmas, as they have 
been called: an upper, highly siliceous or acidic, of low specific 
gravity and light-coloured, and corresponding to such kinds of 
plutonic and volcanic rocks as granite and trachyte ; and a lower, 
less siliceous or more basic, more dense, and more highly 
charged with iron, and corresponding to such igneous rocks as 
the dolerites, basalts, and kindred lavas. It is interesting here 
to note that this conclusion, elaborated by Durocher and Von 
Waltershausen, and usually connected with their names, appears 
to have been first announced by John Phillips, in his “« Geologi- 
cal Manual,” and as a mere common-sense deduction from the 
observed phenomena of volcanic action and the probable results 
of the gradual cooling of the earth.4 It receives striking con- 
firmation from the observed succession of acidic and basic vol- 
canic rocks of all geological periods and in all localities. It 
would even seem, from recent spectroscopic investigations of 
lockyer, that there is evidence of asimilar succession of magmas 
in the heavenly bodies, and the discovery by Nordenskjo!d of 
native iron in Greenland basalts affords a probability that the 
inner magma is in part metallic.® 
(5) Where rents or fissures form in the upper crust, the mate- 
rial of the lower crust is forced upward by the pressure of the 
less supported portions of the former, giving rise to volcanic 
phenomena either of an explosive or quiet character, as may be 
determined by contact with water. The underlying material 
— 2 ee RE Re es ay Se es 
1 Hopkins, Mallet, Sir William Themsen, and Prof. G. H. Darwin main- 
tain the solidity and rigidity of the earth on astronomical grounds; but 
different conclusions have been reached by Hennesey, Delaunay, and Airy. 
In America, Barnard and Crosby, Dutton, Le Conte, and Wadsworth have 
discussed these questions. fh inch 
2 An objection has teen taken to the effect that the supposed ellipsoidal 
form of the equator is inconsistent with a plastic sub-crust. But this ellips- 
oidal form is not absolutely certain, or, if it exists, is very minute. Lonney 
has ina recent lecture suggested the important ccnsideration that a mass 
may be slowly mobile under long-continued pressure, while yet rigid with 
reference to more sudden movements. i P 
3 Phil. Trans., 1884. Also Crosby in Proc. Boston Sec. Nat. Hist., 
1883. — 6 Hone gee 
4 Phillips says (* Manual of Geology,” 1855, p. 493) :—‘* If we regard them 
(the internal crystalline rocks) as acquiring solidification by cooling in zones 
parallel to the surface, we should have sheets of granitic and basaltic rocks 
generated below, the first uppermost, the last undermost, while above the 
several strata were produced in aseries beginningat the bottom. In this sense 
the rocks of fusion may be called, with Lyell, Aypogene. Certainly under par- 
t cular areas of country are found evidence of the liquefaction of one set of 
igneous products after the solidification of others. Many dykes of basalt tra- 
versing granite show themselves to have been in fusion after the solidification of 
the granite."’ In various forms Phillips returns to this idea, as at pp. 556 
and 564, in that unpretending manner which was his wont. Dr. Sterry Hunt 
has kindly directed my attention to the fact of Phillips’s right of priority in 
this matter. Durccher in 1857 elaborated the theory of magmas in the 
Annales des Mines, and we are indebted to Dutton, of the United States 
Geological Survey, for its detailed application to the remarkable volcanic 
outflows of Western America. 
5 These basalts occur at Ovifak, Greenland. Andrews has found small 
particles of iron in British basalts. Prestwich and Judd have referred to 
} the bearing on general geology of these facts, and of Lockyer’s suggestions. 
