^{page 147 note 1} Treatise on Natural Philosophy, Thomson and Tait, p. 721.

^{page 147 note 2} Prof. Helmholtz has calculated, from the rate of cooling of lavas, that the earth in passing from 2000° to 200° C. must have taken three hundred and fifty millions of years! Dana, commenting upon this, says: “But the temperature when the Archæan Time (Laurentian and Huronian periods) ended was probably not over 18° C. (100° Fahr.).” Dana’s Manual of Geology, second edition, p. 147.

^{page 148 note 1} I adopt in all cases the British Association Unit of Conductivity, viz. centimetre—gramme—second.

^{page 148 note 2} This is arrived at by calculating the total increase of heat downwards in both cases and dividing by 3500 feet the total thickness of the strata.

^{page 148 note 3} In these illustrations, and my reasoning throughout, for the sake of simplicity I omit all consideration of specific heats, as they do not affect the line of argument adopted, except that in nature they further complicate the thermal effects.

^{page 149 note 1} British Association Report on Underground Temperature, 1870, pp. 31–2.

^{page 150 note 1} Ibid. 1873, p. 253.

^{page 150 note 2} Ibid. 1875, p. 15.

^{page 150 note 3} British Association Report on Thermal Conductivities of certain Rocks, 1875, p. 57.

^{page 150 note 4} The estimate is really made in terms of the thermal capacity of the unit of bulk, which is averaged at 400; the unit in length being a British foot, and unit of time a year.

^{page 151 note 1} Prof. Judd points out, in his admirable “Contributions to the Study of Volcanos,” Geol. Mag. 1876, p. 211, that in the Southern Tyrol, during the Permian period, “enormous masses of volcanic rock were erupted, leading to the formation of volcanic mountains at least 8,000 feet to 9,000 feet high.” The great “Whin Sill” has also been proved by Topley and Lebour to be a vast intrusive sheet. Innumerable similar examples may be quoted; and as it is usually only by denudation we get a glimpse of those volcanic rocks occupying the interior of the crust, we may be sure there are many others unknown because undisclosed.

^{page 151 note 2} Ramsay, Geological History of some of the Mountain Chains and Groups of of Europe, Mining Journal, 1875. Judd, Contributions to the Study of Volcanos, pp. 133–41.

^{page 151 note 3} As a proof of the failure at present to establish any law of increase of temperature, the remarkable temperature results in the boring at Sperenberg, near Berlin, 4172 feet deep, may be quoted. In this case the first 283 feet were in gypsum, with some anhydrate, and the remainder entirely in rock-salt. Although rock-salt possesses a considerably higher conducting power than even quartz, being ·01154, and more than twice that of Aberdeen granite, the rate of increase of temperature averaged 1° per 51·5 feet. (British Association Report on Underground Temperature, 1876, p. 206.) If the rates of increase should be inversely as their conductivity, on the supposition of an uniformly cooling globe, the increase of temperature should, compared with Mont Cenis, not be more than 1° per 200 feet. Does it not suggest that the source of the heat must be nearer the surface, or that there exists beneath, some mass of rock abnormally heated?—a supposition consistent with the geological fact already stated of the intrusion of heated matter into the crust of the globe and at various epochs of its history.

^{page 151 note 4} Log Letters of the Challenger.

^{page 152 note 1} The Moon as a Planet, as a World, and a Satellite, Nasmyth and Carpenter.

^{page 152 note 2} Mr. Mallet has controverted Nasmyth’s conclusions, Proc. Roy. Soc. 1874, p. 366. Nevertheless, he admits the fact of certain pieces of cold cast-iron floating upon molten iron.

^{page 152 note 3} In my Presidential Address to the Liverpool Geol. Soc. Session 1875–6, “On the Moon and the Earth,” it is suggested, p. 89, that “the sediments accumulating on the Earth from age to age must act as a non-conducting envelope.”

^{page 153 note 1} Mr. Croll (Climate and Time) attempts to explain why the secular cooling of the Earth does not affect the bottom water of the Ocean, but the explanation comes after the knowledge of the fact.

^{page 153 note 2} “On Geological Time,” Presidential Address, Liverpool Geol. Soc. Session 1876–7.

^{page 153 note 3} Sir W. Thomson, however, infers, that as the Sun must have been hotter in former ages, the atmospheric agencies were then more potent; but this is all pure hypothesis, no proofs that they were being adduced. Mr. Croll again has his own hypothesis on the subject, but the fact is we know absolutely nothing of the Sun’s heat, and cannot safely reason on conjectures.