net. On calm warm days in summer and autumn-dreamy days are the best-a bag of fine muslin fastened to a stout ring of wire, and towed at the stern of the boat, will gather a rich harvest of beautiful forms. The student will have a fair chance of obtaining some specimens in which the ova are far advanced towards maturity, and may possibly be fortunate enough to keep them until the embryos are discharged. If he can rear these into polypites he may complete an imperfect life-history, and fill up one of the many gaps in our knowledge of the tribe. This has been done in a few cases; by care and patience it might no doubt be done in more. And even those who are not prepared for any profound study of the structure and history of the medusa may find the townet a not uninteresting resource during their visits to the sea; and if they have any sensibility to beauty in their souls, can hardly fail to derive the highest gratification from watching the exquisite forms and movements, and the singular habits of these "animated bubbles." DESCRIPTION OF PLATE LXXXVIII. FIG. 1. Crematostoma flava, A. Agassiz. 7. A Campanularian medusa, not yet described (?). Oceania languida, A. Agassiz (the young). Probably the gono- The same, in the adult state. Cladonema radiatum, Dujardin. The medusa in an early stage of its development. 8. Clavatella prolifera, Hincks. The ambulatory zooid. 9. Dicoryne conferta. Alder. The female natatory zooid; an ovum, with its germinal vesicle and spot, is visible within the cavity of the body. 10. The Hydroid planula or embryo. [Figs. 1, 4, 5, and 6, are after Alexander Agassiz; fig. 9, after Allman; and the rest from drawings by the Author.] 350 THE FIRST CHAPTER OF THE GEOLOGICAL RECORD. BY DAVID FORBES, F.R.S., &c. HE term Geology, which signifies the science of the earth, being derived from the two Greek words, T, "the earth," and Aóyos, "a word or argument," has been variously interpreted by different writers on the subject.* During the last generation, geology, as a science, was studied altogether from a purely mineralogical and physical point of view; an interpretation which became completely reversed when the introduction of paleontology, called in as an aid to its study, so absorbed the attention of the majority of geologists, to the exclusion of almost all other branches of the science, that most of the later works on geology, especially here in England, may be regarded rather as histories of the development of life upon our globe than treatises on its geology in its more extended sense. A perusal of most, even of our best-known manuals of geology, will show that their contents are almost entirely devoted to the fossiliferous strata, commencing their descriptions either with the most recent formations, and proceeding backwards until they stop at those more ancient ones, in which only traces of organic remains have as yet been discovered; or vice versâ, beginning with the lower Silurian or Cambrian rocks or in later years (since the discovery of that most perplexing organism the Eozoon Canadense) with the Laurentian formation, and treating the others in ascending order up to the present time: a system, which in either case makes the student feel the evident want of a beginning or first chapter in the geological record, whilst at the same time it imposes, as it were, a dictatorial boundary to his field of research in a similar manner to what it would be, if he was told, when studying Including the strangely inappropriate application of the term by M. Meunier, who writes of the "Geology of the Heavens!" and has lately published a work entitled "Le Ciel géologique." Paris, 1871. ethnology or the history of mankind, to ignore everything connected with the subject before printed records existed, or as if an astronomer was advised to discard all discoveries of which he had not tangible evidence as to their correctness. Just, however, as the recent advances of the collateral sciences have cleared up so many difficulties, and have added so much to our knowledge of prehistoric times, and the condition of the human race in earliest periods, or of parts of the cosmical system, which the astronomer of old could never even have imagined to be within man's power of investigation; so it is to be expected, with the aid of our daily improving information and appliances, that proportionate advances may also be made in our knowledge of what may be termed the prozoic history of the earth; that is to say, of the different stages through which our globe has passed before it became fitted for the habitation of organisms even so low in the scale of life as are met with in the previously mentioned formations, which modern geologists appear so often to regard as the very ultima thule of their investigations. On the present occasion it is proposed to make an attempt to sketch out such an introductory chapter in geology as is here referred to, premising, however, that from its very nature it cannot be other than in the highest degree theoretical, and must be regarded only as an essay, in which the more recent discoveries in physical and chemical science are appealed to in elucidation of a subject which, without their aid, would be all but unapproachable; and this is here brought forward in the belief that attempts made from time to time, to generalise and put into shape the somewhat disconnected facts and observations relating to this subject, cannot but do good, notwithstanding that it must at the same time be self-evident that the views herein expressed will require to be modified from time to time, according as the progress of scientific investigation furnishes more reliable and extended data for generalising upon than are in our possession at the present moment. As is well known, even the most ancient philosophers entertained the opinion that our globe had not always been what it was in their age; that it had passed through varied phases, and that it once upon a time had even had a commencement to its present career. Later on, when astronomy came to be studied as a science, astronomers went still further, and reasoned from a consideration of the earth's form, &c., that it must at a remote period have been in a fluid, or at least plastic condition; a result which the subsequent observations on the temperature of the earth in depth and the products of volcanic action confirmed, and led to the conclusion that our globe must once have been a sphere of molten matter, which had solidified on its exterior, owing to the cooling action of the surrounding air. The celebrated Laplace went still further, and, from a consideration of Herschel's researches on nebula, propounded his so-called nebulous theory of the earth's origin, according to which our sphere owed its existence to the aggregation and condensation of nebulous matter. The state of the natural sciences of the period was not, however, sufficiently advanced to furnish means by which this theory of Laplace could be either confirmed or disproved, so that it was long looked upon as a visionary hypothesis which was never even imagined as likely to be so far confirmed by future discoveries in science, as to become at this moment the most plausible explanation of a beginning of our world which has as yet been put forward. This being the case, our chapter of genesis commences by assuming the nebulous theory of the origin of our globe as the starting-point; and the first stage in the history of the earth is consequently the act of aggregating or segregating the nebulous matter in space or, in other words, of gathering together in a gasiform condition the chemical elements of which the earth, with its surrounding atmosphere, is actually composed of. The consequence of the coming together of these elements would, as chemistry teaches us, result in their reacting upon one another with intense energy, giving rise to the development of both light and heat, and forming numerous chemical combinations, the nature of which would be dependent upon the mutual affinities of the elements themselves, and the relative proportions in which they were respectively present in this admixture of gases and vapours. The more simple or binary compounds would naturally be formed first, such as the oxides, sulphides, chlorides, &c.; but these in turn would combine inter se producing salts and other compounds, amongst which the silicates played a very prominent part. The final result of this great display of chemical energy would be to change entirely the nature and appearance of the original nebulous gathering of gasiform matter, for as soon as the chemical action had come to an end, by far the largest proportion of the newly-formed substances would no longer be able to retain the gasiform condition at the lower temperature which then ruled, and would be condensed into fluids, when the whole would assume the shape of a sphere of molten matter surrounded by an intensely heated atmosphere of such of the other compounds and free elements as could still remain volatile at this temperature. This period might be termed the second stage in the history of the earth, and if examined into more closely, it would be found that neither the molten sphere nor the atmosphere surrounding it was of uniform character throughout; but owing to both of them being made up of a number of dissimilar substances, the first impulse of the newly-formed compounds would be to obey the laws of gravity by arranging themselves more or less completely in strata, or more correctly speaking, zones, in accordance with their respective densities, and the study of the composition of the rocks now forming the earth's exterior, and of those brought up from its depths by volcanic forces, along with that of the relative specific gravities of the parts accessible to our observation as compared to the density of the earth's mass as a whole, leads to the deduction that the molten sphere might at this period of its history be regarded as composed of some three great zones (probably with subzones), having the following general mineralogical characters: 1st. An exterior of molten rock of comparatively little density which consisted of silicates, in which an excess of silica was to a great extent combined with alumina and alcali, but containing very little of the other bases, such as lime, magnesia, oxide of iron, &c. 2nd. A middle zone, also of molten rock (silicates), considerably heavier than the former, and in which the silica, present in minimum proportion, existed in combination with a large amount of the bases, lime, magnesia, oxide of iron, and alumina, with but comparatively little potash, and, 3rd. A central nucleus of very much greater density and of metallic nature, the outer part consisting of compounds of the heavy metals with sulphur, arsenic, &c., whilst in the centre the metals themselves are probably in a free state, or as alloys. The constitution of the sphere of molten matter as thus arranged would now present a general character of stability maintained even after its solidification, due to the loss of heat radiated from its surface, and the cooling action of the external air, had commenced. In the atmosphere, however, the arrangement of the gases and vapour in zones would be much less permanent, as by degrees the zones would be more or less broken up by the tendency which gases have to diffuse themselves throughout one another, as well as the condensation in succession of the different vapours contained in it, in proportion as the temperature of the whole became more and more lowered. In the first instance, however, that stratum of the atmosphere next to the earth would be composed of dense vapours of such compounds as are only volatile at very high temperatures, amongst which several of the chlorides, and especially the chloride of sodium or common salt would be most prominent; above this a great zone of carbonic acid gas would prevail, then one of nitrogen with possibly the admixture of some oxygen |