THE speaker began by commenting on the sailing of H.M.S. 'Challenger' in December, 1872, the nature of her mission, and the completeness and perfection of her equipment, under the auspices of the late First Lord of the Admiralty, Mr. Goschen.
After referring to the course laid down for the 'Challenger,' and her expected return in the spring of 1876, he said that her work is rather more than half done, and that important results had been already obtained, some of which would be especially dealt with in his discourse; but it was necessary, first, to give a sketch of the state of our knowledge on the subject in 1872, in order to do justice to the Challenger's' work, without over-estimating its value.
The first successful attempt to ascertain the nature of the bottom of the sea, and of the limit of life at depths greater than five or six hundred feet, was made by Sir John Ross in 1818, who, by the help of a very ingenious machine of his own contrivance, brought up several pounds of mud from 1050 fathoms, or 6300 feet, or about a mile and a quarter, in Baffin's Bay, 72° 30' N., 77° I5' W. The nature of what was brought up was described by Ross and by Sabine, who accompanied him. It was a fine greenish mud, but no accurate determination of the nature of this mud was made. Ehrenberg, in 1853, examined the surface scum and the mud obtained by Penny in 73° and 74° N., and found it to consist of
Diatoms (vegetable), which live at the surface.
Radiolaria (animal), which live at the surface.
Sponge spicules (animal), which live at the bottom.
 In 1854, Bailey determined the nature of the mud procured by Brooke in 900-2700 fathoms (5400-16,200 feet), in the Sea of Kamschatka. There were absolutely no calcareous organisms in the mud; it was purely silicious. These and other observations in the north circumpolar area tend to establish the existence of a circumpolar zone of silicious deposit in the Arctic regions, within the parallel Of 55° Na North Polar silicious cap.
The speaker next alluded to the despatch of the Antarctic expedition under Sir James Ross, in 1839, and the awakening of attention to the importance of the work done by minute organisms by Ehrenberg, in 1836 and 1838. Ehrenberg discovered that organisms, whose skeletons resemble those which occur in cretaceous and tertiary rocks, and sometimes constitute their whole mass are still living. The observations of Sir James Ross and Dr. Hooker, at two distant points of the Antarctic zone, proved the existence of a South Polar silicious cap.
The speaker then noticed the discovery of the nature of the sediment forming the bottom of the deep sea in the intermediate zone (about 110° of latitude), of calcareo-silicious deposit. This dates back to 1853, and is due to Ehrenberg's examination of the soundings brought up by Berryman from 2000 fathoms (12,000 feet), between Newfoundland and the Azores. They consisted of silicious Diatoms, Radiolaria, and Sponge spicules, as in the Arctic and Antarctic seas: but the great mass of the deposit was formed of calcareous Foraminiifera. On these observations Ehrenberg based the conclusion then enunciated, that "chalk is nothing but a heap of dead foraminiferal skeletons."
Professor Huxley then alluded to the confirmation of these results by Bailey, himself, and others, and to the discovery of the extension of a similar deposit over the South Atlantic and into the Indian Ocean; and the further confirmation on a great scale by the 'Challenger.'
Having mentioned Ehrenberg's opinion, that Globigerinæ, &c., live at the bottom, and the evidence adduced by Dr. Wallich in confirmation of that view, he referred to Major Owen's results, and to the 'Challenger's' confirmation of the affirmative proposition that they certainly live at the top; he added, that the negative proposition that they do not also live at the bottom is left open. The fact that animals which do live at the bottom feed on the Globigerinæ is probably of no great weight.
The speaker next adverted to the Greensand deposits of various geological ages, the nature of which was discovered by Ehrenberg in 1854, and to the nearly contemporaneous discovery of recent greensand (100-300 fathoms) by Pourtales (1854), and the investigation of  its origin by Baily in 1856; while Parker and Jones subsequently ascertained the formation of greensand to be going on in the Australian seas. The 'Challenger's' discovery of greensand at the bottom of the Agulhas current (200-300 fathoms) was then mentioned. Thus far the value of the work of the 'Challenger' lies in extending and confirming conclusions already more or less established. The next matter is especially her own.
The barrenness of the deeper parts of the Mediterranean, and the existence of a fine clay there, was already known ; but no one suspected the existence of extensive deposits of barren clay in the middle of the great calcareo-silicious zone in open ocean.
The dredgings of the 'Challenger' have recently shown that certain deep oceanic valleys contain thick deposits of finely-divided red clay, composed of a silicate of alumina and peroxide of iron. Thus, between Teneriffe and S. Thomas a great valley was found at a depth of about 18,000 feet, the bottom of which was covered with this red material. The origin of such deposits is suggested by the results of some experiments by Mr. Buchanan, the chemist to the 'Challenger,' who, by treating the Globigerina-marl with dilute acids, obtained about 1 or 2 per cent. of insoluble residuum, which strikingly resembled the clay in question. Hence the conclusion drawn by Professor Wyville Thomson, that the great deep-seated deposits of red clay actually represent the remains of marine organisms, of which myriads must have suffered decomposition to furnish these vast accumulations of their debris. If this conclusion be correct, not only silicious and calcareous, but arginaceous deposits may be formed by the long-continued action of vital agencies. And as, by well-known processes of metamorphosis, such silicious, calcareous, and arginaceous deposits may be converted into all the chief kinds of rock which constitute the earth's crust, it will follow that rocks of almost any mineralogical composition may be directly or indirectly generated by the action of living organisms. Having discussed various explanations and dismissed them as all at present unsatisfactory, the speaker said that the certainties are:
1. That, beyond certain depths, the calcareous organisms which must fall over the area covered by the ocean disappear, and their place is taken by fine red clay.
2. That when the ordinary Globigerina ooze has its calcareous matter removed, a residuum of fine red clay remains.
But standing firmly on the basis of bare facts, as they are now ascertained, it is permissible to use these facts as a basis for  deductions. Suppose the globe of the earth to be covered with water uniformly to a depth of two thousand fathoms (twelve thousand feet, or about two miles), the merely tidal and current movements which would then exist in the water would be insufficient to cause any important amount of degradation of the solid crust, and there would be no sedimentary deposits worth speaking of.
Now let the lowest forms of vegetable and animal lifeDiatoms, Radiolaria, Foraminifera, and Spongesbe introduced, and the result would be the formation of the silicious pole caps, and of the calcareo-silicious intermediate zone; and these might accumulate until they formed beds of rottenstone and of chalk, of many thousand feet in thickness.
By well-known metamorphic processes the rottcnstone might be converted into a dense opaline rock, or perhaps into quartzite; while the chalk might be converted into crystalline limestone; and in each of these all trace of its origin might disappear.
Next let parts of the bottom in the calcarco-silicious area be raised to within 1000 feet, and others depressed to 18,000 feet. Judging by our present knowledge, the former might be replaced by greensand, and the latter by red clay. The greensand miglit be metamorphosed into endless varieties of those minerals into which silica, alumina, iron, and potash enter in diverse proportions; the clay into shales, schists, slates, and even into gneiss and granite.
And thus our imaginary world would be covered with more or less extensive patches of all the most important rocks which enter into the composition of the globe, every particle of which would at one time have formed a part of a living organism; though in some, no trace of this primitive composition would remain, and though it is quite possible that all traces of organic life might be obliterated in every one of them.
Application of these results was then made to the reconciliation of an apparent discrepancy between biological theory and geological fact.
1. There is conclusive evidence that the present species of animals and plants have arisen by gradual modification of pre-existing species.
2. There is conclusive evidence that, even geologically speaking, the process has been very slow, and has continued over a much longer period than that which corresponds with the fossiliferous rocks.
3. Nevertheless, beneath the fossiliferous rocks lies a great thickness of unfossiliferous quartzites, sandstones, slates, gneiss, and granite, almost entirely azoic.
 These researches consequently lend great support to the views of those geologists who find an explanation of the past history of the rocks in the present operations of natureviews which were held half a century ago by Sir Henry De la Beche, were advocated by Mr. Poulett Scrope, and were still more clearly developed by Sir Charles Lyell, who has so ably elaborated the doctrine of uniformitarianism originally enunciated by Hutton. Sir Charles has, indeed, survived the prejudices which at first opposed his views, and has lived to see most of the reputed heresies of his youth become established as the creed of every philosophical geologist.