fishes related to the recent Cestracion, or Port Jackson shark, are found in some beds of the mountain-limestone, especially near Bristol, and at Armagh, in Ireland. At the latter place Admiral Theobald Jones has collected vast numbers of beautiful specimens of teeth (both of the cutting and the crushing forms), and of fin-bones or spines from the hard grey marble; some of them being of large size.*

48. REPTILES OF THE CARBONIFEROUS PERIOD.-Not many years since, reptilian remains not having been then found in the carboniferous rocks, it was supposed that the earliest date of the existence of reptiles on this earth was that of the Permian rocks. Now, however, we have both bones and foot-tracks of reptiles not only in the coal-measures and in the mountain-limestone, but in the Devonian strata.

The Archegosaurus,† a sauro-batrachian of close relationship with the Labyrinthodon (see p. 552), has left abundant remains in the coal-fields of Rhenish Prussia,‡ the Parabatrachus § occurs in the coal of Scotland; and the allied Dendrerpeton,|| and the labyrinthodontoid Baphetes,¶ have been found in the coal-measures of Nova Scotia.

Cheirotherian foot-tracks of reptiles have been discovered on some of the shales of the coal-measures in the United States; ** and in the red shales at Pottsville, Pennsylvania, which Dr. Isaac Lea refers to the carboniferous series, but which the State-geologists, the Professors W. B. and H. D.

* A very magnificent series of these fossils has been presented to the Geological Society's Museum, by Admiral Jones, F.G.S.

† Medals, p. 745.

See Hermann von Meyer's elaborate Memoir on the Archegosauri and allied genera, in the Palæontographica, vol. vi.

Quart. Journ. Geol. Soc. vol. ix. p. 67.

|| Ibid. p. 58; and Lyell's Manual, 5th edit. p. 405; Medals, p. 746. ¶ Quart. Journ. Geol. Soc. vol. x. p. 207, and vol. xi. p. 8.

** See above, p 570; and Lyell's Manual, 5th edit. p. 401.

Rogers, determine to be the local representative of the lower portion of the Mountain-limestone, both Dr. Lea and Prof. H. D. Rogers have found foot-tracks resembling those of thin-toed saurians.*

Similar tracks to the last mentioned have been found on the coal-shale of the Forest of Deane; and Cheirotherian footprints on a gritty carbonaceous stratum, forming a "roof" of the coal, in one of the coal-mines at Dalkeith,† which are worked in the Mountain-limestone series.

Here then are evidences enough, and doubtlessly further researches will contribute more, of the terrestrial and amphibious vertebrated animals, which, crawling among the tangled swampy jungles, and on the oozy surface of the shores, inhabited the lands that were clothed with the luxuriant vegetation of the carboniferous flora.

49. CLIMATE OF THE PALEOZOIC AGES.-The cause of the difference between the natural climates now prevailing over extensive zones of the earth's surface, and those which the organic remains discovered in many of the older strata lead us to conclude have formerly subsisted during very long periods of time-and apparently over the greater part of its whole extent is one of those geological problems the solution of which is not at present wholly within our reach. Unable to account for such a distribution of an apparently high climatorial temperature—a diffusion of heat and light so greatly at variance with that which has prevailed during the human epoch-the mind naturally endeavours to penetrate the mystery by a reference to physical causes extraneous to our planet. But, as yet, astronomy has afforded no satisfactory elucidation of the subject.

A variation in the eccentricity of the earth's orbit, and a change in the position of the tropical zone, on account of the precession of the equinox

Sauropus primævus, Lea, Transact. Amer. Phil. Soc. vol. x. 1852. Hugh Miller's Testimony of the Rocks, p. 78. These specimens are deposited in the Museum of the Geological Survey, in Jermyn Street.

-both changes, which, though extremely slow, are appreciable-have been brought forward to account for the phenomena under review. From the diminution of the eccentricity of the earth's orbit round the sun, by which the ellipse is in state of approach to a circle, the annual average of solar radiation is on the decrease; and therefore, as a general cause, and one affecting the mean temperature of the whole globe, and the effect of which is both inevitable and susceptible of exact estimation, it is deserving consideration.*

In assuming a temperature in northern regions sufficient to support a quasi-tropical vegetation, it must, too, be borne in mind, that light is as indispensable as heat for the luxuriant growth of tree-ferns, conifers, cycadeæ, &c.; and, by analogy, for the gigantic club-mosses and ferns of the carboniferous period. The absence of light for weeks or months would probably be fatal to most of the existing tropical forms of vegetation. It is therefore as necessary to account for the presence of light as for a high temperature in the northern regions, where fossil plants indicate the former genial influence of a warm climate and sunny skies during the carboniferous era.

To account for the existence of regions capable of supporting such a flora as that of the coal-measures in northern latitudes, it has been argued by an American author,† that the changes on the earth's surface which have produced the successive strata and organic remains, as far as these are regular, are attributable to the progress of the perihelion point around the ecliptic: that by the precession of the equinoxes, and the progress of the perihelion-rotation of the earth's orbit, a great uniform zone of tropical climate, which formerly surrounded the globe in a different course from that of the present tropics, has by very slow degrees changed, its position, and that the present tropical zone succeeds it in a continued change of position.

* Sir J. F. W. Herschel, on the Astronomical Causes which may influence Geological Phenomena. Geol. Trans. 2nd ser. vol. iii. p. 293. +"An Essay on Organic Remains as connected with an ancient Tropical Region of the Earth." By T. Gilpin, Philadelphia, 1843.

The subject of hypothetical causes of conditions and changes of temperature in former periods of the earth's history has been also treated of by the late Mr. W. D. Saull, in his " Essay on the Connexion between Astronomical and Geological Phenomena," 8vo, London, 1853; and by Mr. Evan Hopkins, in his book entitled, "On the Connexion of Geology with Terrestrial Magnetism." 8vo, London, 1851.

Whether either or both of the above-named causes may be regarded as applicable or adequate to have produced any of the contemplated effects, I must leave to the astronomers to determine. There is, however, another cause, first suggested by Sir C. Lyell, that possesses all the essential requisites of a vera causa; and that is the varying influence of the distribution of land and sea over the surface of the earth.*

A change of such distribution in the lapse of ages, by the degradation of the old lands, and the elevation of new, is a demonstrated fact; and the influence of such a change on the climates of particular regions, if not of the whole globe, is a perfectly fair conclusion, from what we know of continental, insular, and oceanic climates by actual observation. "Here, then," observes Sir John Herschel, "we have, at least, a cause on which a philosopher may consent to reason; though whether the changes actually going on are such as to warrant the whole extent of the conclusion, or are even taking place in the right direction, may be considered as undecided, until the matter has been more thoroughly examined." Another astronomical source of periodical variability in the general temperature of the earth's surface has been brought forward by Sir John Herschel,‡ who, instancing the observed variability in the luminosity of certain stars, suggests the possibility that the sun of our system may in the course of ages be subject to similar phases of augmented or diminished energy; and that such variability in the periods contemplated by Geology may have given rise at one epoch to a general equatorial climate, and at another to one far below the general temperature that now prevails.

Two other supposed causes of the existence of a general warm climate in the earlier geological periods may be here mentioned: namely, 1. Central heat, supposed to be the remains of a former and very much greater heat, which has been gradually diminishing during some indefinite period of time: 2. The passage of the solar system through some region

* Highly important remarks on the relative extent of land and sea in the paleozoic ages were found in the 18th chapter of Sir Roderick Murchison's "Siluria."

"Discourse on the Study of Natural Philosophy," p. 146.

In his magnificent work, entitled, "Result of Astronomical Observ. ations made at the Cape of Good Hope," 1847, p. 351.

of stellar space, of which the temperature, owing to stellar radiation, is much greater than that in which it is now placed.

Both of these hypotheses have been treated, especially in reference to the Glacial period in the Tertiary age, in a masterly manner by Mr. W. Hopkins, of Cambridge, in the Geological Society's Journal. With regard to the first, he remarks, that the effect on the superficial temperature due to this cause may have been formerly of any amount, but is now reduced to within 1-20th of a degree of Fahrenheit of that ultimate limit to which it would be reduced in an indefinite period of time, supposing the external conditions under which the earth is now placed-such as the amount of radiation from the sun and stars, and the state of the atmosphere-to remain as at present. Poisson has calculated that it would require 100,000 millions of years to reduce the present temperature by about 1-40th of a degree of Fahrenheit. It is probable, therefore, that many millions of years must have elapsed since the central heat can have elevated the earth's superficial temperature by a single degree, and it is only to the more remote geological periods that we can refer for any very sensible change in the climatal conditions of our globe due to this Prof. Ramsay, following out this line of argument, has shown † that the internal heat of the earth has exerted no important climatal influence during any of the geological periods, from the Silurian times downwards.

cause.

Of the second hypothesis, which was suggested by Poisson, Mr. Hopkins remarks that it involves the necessity of supposing a totally different distribution of the group of stars to which the sun should belong, or the near approach of the solar system to some individual star; either supposition being probably inconsistent with the integrity of the solar system as it now exists. At all events, this hypothesis would not account for the changes of climatal temperature that have taken place in comparatively late geological times.

The soundness of the theoretical views of the probable causes of climatal changes suggested by Lyell is well elucidated by Mr. W. Hopkins, in the Memoir above quoted, by an exposition of the probable configuration of the European and North American areas in the Glacial period; when, according to Mr. Hopkins, the absence of the Gulf-stream, with its influences upon the western coast of Europe, may be assumed,

* See his Memoir, "On the Causes of Change in the Earth's Superficial Temperature," Journ. Geol. Soc. vol. viii. p. 56; and his Presidential Address for 1852, ibid. p. lvii.

+ Quart. Journ. Geol. Soc. vol. xi. p. 203.