
7 a Py A ™ rT vi ee +, , +. 
1 i ee q “ 
50 | _ GEOGNOSY. Book Fs 
1760° Fahr, ; at 50 miles it must be 4600°, or far higher than the — 
fusing-point even of so stubborn a metal as platinum, which melts at — 
3080° Fahr.t. (b.) All over the world volcanoes exist from which — 
steam and torrents of molten lava are from time to time erupted. 
Abundant as are the active volcanic vents, they form but a small | 
proportion of the whole which have been in operation since early 
geological time. It has been inferred therefore that these numerous — 
funnels of communication with the heated interior could not have 
existed and poured forth such a vast amount of molten rock, unless — 
they drew their supplies from an immense internal molten nucleus. 
(c.) When the products of volcanic action from different and widely- — 
separated regions are compared and analysed, they are found to 
exhibit a remarkable uniformity of character. Lavas from Vesuyius, 
from Hecla, from the Andes, from Japan, and from New Zealand 
present such an agreement in essential particulars as, it is contended, 
can only be accounted for on the supposition that they have all 
emanated from one vast common source.” (d.) The abundant 
earthquake shocks which affect large areas of the globe are main-— 
tained to be inexplicable unless on the supposition of the existence — 
of a thin and somewhat flexible crust. These arguments, it will be 
observed, are only of the nature of inferences drawn from observa- 
tions of the present constitution of the globe. They are based on 
geological data, and have been frequently urged by geologists as 
supporting the only view of the nature of the earth’s interior 
compatible with geological evidence. 7 
2. The arguinents against the internal fluidity of the earth are 
based on physical and astronomical considerations of the greatest 
importance. They may be arranged as follows :-— ae 
(a.) Argument from precession and nutation.—The problem of 
the internal condition of the globe was attacked as far back as the 
year 1839 by Hopkins, who endeavoured to calculate how far the 
planetary motions of precession and nutation would be influenced by 
the solidity or liquidity of the earth’s interior. He found that the 
precessional and nutational movements could not possibly be as they 
are if the planet consisted of a central core of molten rock sur- 
rounded with a crust of twenty or thirty miles in thickness, that 
the least possible thickness of crust consistent with the existing 
movements was from 800 to 1000 miles, and that the whole might 
even be solid to the centre, with the exception of comparatively small 
vesicular spaces filled with melted rock.® 
M, Delaunay,’ threw doubt on Hopkins’s views, and suggested 
* But Sir W. Thomson has shown that while the rate of Inctiease of. temperature is 
probably 1° for every 51 feet for the first 100,000 feet, it will beein to diminish below 
that limit, being only 1° in 2550 feet at $00,000 feet, and then rapidly lessening. Trans, 
toy. Soc, din, xxiii., p. 163. 
* See D. Vorbes, Popular Seience Review, April 1869. 
* Phil. Trans, 1839, p. 881; 1840, p. 193; 1842, p. 43; Brit. Assoe. 1847, 
* Ina paper on the hypothesis of the interior fluidity of the globe, Comptes-rendus, 
July 13, 1868. Geol. Mag. v. p. 507. ' See also a paper by H, Hennessy: -ren 
6 March, 1871, and Geol. Mag. viii. p. 216, re _ ys Co So ; 
