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554 REPORT—1904. 
in solution two types of waves will be propagated through it, with different 
velocities. Tremors will first arrive, due to the compressibility of the magma, 
and subsequently waves, caused by the extrusion of gaseous vesicles, due to the 
changes of pressure. If my argument is valid, that for solidity loses its force." 
I will now give my reasons for thinking that the substratum, if a liquid, is not 
a still liquid, but is affected by convection currents. 
Availing myself of Sir Arthur Riicker’s observed values of the melting- 
temperature and specific heat of Rowley rag, I have calculated that if the 
substratum of the crust be a still liquid, the thickness of the crust comes out 
22 miles, and the corresponding time since it began to solidify about eight million 
years. This is a much shorter time than geologists would admit. This result 
proves that the substratum is not a still liquid, and must, therefore, be affected by 
convection currents, bringing up heat from below and delaying the thickening 
of the crust. The existence of convection currents being thus, as I submit, 
established, I will add my reason for believing that they ascend beneath the 
oceans, 
By a somewhat complicated calculation, which, although assailed by Mr. 
Blake,’ has been ably defended by Mr. Brill,® I have, I think, proved that the 
substratum beneath the ocean is less dense than beneath the land. This shows 
that the upward currents are beneath the ocean. I have at the same time proved 
that the sub-oceanic crust does not reach quite so deep down as the continental 
crust, and thet its upper layer is thin and very dense; from which I infer that it 
consists of basic lava flows, the oxydation of which would afford the red clay 
which covers the bottom of the deeper oceans, 
These convection currents, ascending beneath the ocean, and then flowing 
horizontally towards and beneath the continents till they descend, are, in my 
opinion, the cause of the compression of the continental crust. 
At the request of the President, Professor T, McKunny Hueuss explained the 
position and arrangement of the specimens exhibited in the museum in illustration 
of the subject under discussion, 
They were, as far as possible, arranged to show the ‘nature’ of earth move- 
ment and the relation between superinduced structures, which might suggest 
their ‘origin.’ Great continental folds, and local readjustments in connection 
with them, may be exemplified even in small specimens. From these we should 
infer, as we should also from observations over large areas, that faults and folds 
were generally due to movements in relief of lateral pressure, while faults due to 
the dropping of the marginal portion of uplifted and exposed areas are com- 
paratively rare. What the nature of the movement will be in various regions 
would largely depend upon the character of the rocks affected. Heim had 
estimated the probable breadth of the Alps if the strata were pulled out flat. 
Similar calculations had been made among the folded mountain ranges of 
America ; but, as our specimens show, all rocks were not susceptible of the same 
kind of compression. One rock had gained in vertical thickness at the expense of 
horizontal extension by molecular rearrangement of the particles, which had, or 
could assume, a flattened form at right angles to the direction of the pressure. 
Another rock which would not yield to this kind of readjustment must fold; but 
faults and crumplings occur along belts, and are in easement or relief of strain 
along lines of less resistance during movements of uplift and depression. 
In the production of folding without fracture time is an element, and tempera- 
ture must be taken into account. When the rocks do break, earthquakes record 
the rent, and volcanic phenomena follow the relief of pressure on the superheated 
rocks. If we could explain the great epeirogenic movements by a shrinking of 
the interior and a tangential pinching of the hardened outside crust, we should 
establish a vera causa which would involve the crumplings along limited belts, 
where vertical expansion was produced in compensation for horizontal extension. 
Nature is full of automatic compensations and conflicting forces, and in this. dis- 
1 Proceedings of Cambridge Phil, Soc., vol. xii., 1904. 
2 Phil, Mag., 1894. 5 Phil, Mag., 1895, 
