I.—The Climate Controversy

THE GEOLOGICAL MAGAZINE. NEW SERIES. DECADE II. VOL. III.

No. IX.—SEPTEMBER, 1876.

ABTICLBS.

I. — THE CLIMATE CONTROVERSY. By SEARLES V. WOOD, JUN., F.G.S. rPHE cause of changes in Climate during past Geological periods _|_ having lately become, and being likely to continue, a prominent topic of interest, I have attempted here to bring together the more prominent features in the controversy, under the idea that to many readers, who have not either the time or the inclination to more closely examine the subject, this may be acceptable. In so doing I have, with one partial exception, confined myself to discussing the difficulties which beset all the various theories offered as a solution, rather than attempted the advocacy of any one of those theories in particular. The following seven causes have been advanced, at various times, to account for the changes in Climate, which the evidence of the Geological record proves have taken place during that portion of the Earth's history which it covers. 1. A decrease in the original heat of our planet. 2. Changes in the obliquity of the ecliptic. 3. The combined effect of the precession of the equinoxes, and of the excentricity of the Earth's orbit. 4. Changes in the distribution of land and water. 5. Changes in the position of the Earth's axis of rotation. 6. A variation in the amount of heat radiated by the Sun. 7. A variation in the temperature of those regions of space through which the solar system has moved. No. 1.—Since all physicists are agreed that the earth has under- gone a process of cooling, the main questions that have arisen under this head are: what has been the effect of this cooling upon the temperature of the earth's surface, and whether evidences of such refrigeration can be traced in the Geological record ? As to the first of these questions, a celebrated physicist is of opinion that the influence of the earth's proper heat on climate is not only inappreciable at the present day, but must always have been so since the condition of the crust allowed of the existence of life upon the earth. As to the second, it must be admitted that organic remains throw but little light upon it. Beyond the fact that at the present day reptile-life cannot exist in very cold climates, and seems to prevail in the direct ratio of the warmth of the air or water in which it lives, there is but little to show that the climates of those DECADE II. —VOL. III.—NO. IX. 25

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 09 Aug 2017 at 18:33:27, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800155190 386 Searles V. Wood, Jun.—The Climate Controversy. earlier periods in which that form of life abounded were warmer than the climates of the Tertiary group. So far as the lower types of animal-life and the remains of vegetation furnish evidence, the climate of Northern Europe during both the Eocene and Miocene periods would seem to have been as warm as that of any preceding period. Though so little evidence is thus, however, afforded by organic remains, there is one fact that seems to me to bear upon the subject which I have not seen alluded to, and it is this, viz. the effect of increased heat over the earth's surface generally would, as is well known, produce increased rainfall, and, as a consequence, more rapid atmospheric denudation; more sediment, that is to say, would be carried down by rivers to furnish the material for sedimentary deposits accumulating around continental areas. Now if we compare such groups of rock as the Silurian with Tertiary groups, we are struck with the smaller amount of change presented by the forms of organic life in any given thickness of sedimentary deposit, in the case of the older, than in the case of the newer formations; and, ceteris paribus, this would raise a presumption that sedimentary deposit proceeded more rapidly during the earlier Geological periods than it did during the later. No. 2.—It is obvious that no alteration in the obliquity of the ecliptic would affect the quantity of heat received from the Sun by the Earth as a whole. The effect, whatever it may be, would be confined to the distribution of this heat in latitude. There appears to be a unanimous opinion among astronomers up to the present time, that the utmost limit within which the inclinations of the planes of the equator and ecliptic to each other can vary is less than a degree and a half;1 a quantity that seems too little of itself to have affected climate to any great extent, whatever be the correct view as to the kind of alteration which such variation would produce. Nor would a decrease in the obliquity remove the difficulty offered by long seasons of continuous night, which some geologists consider irreconcilable with the fossil vegetation of Spitzbergen, unless it were so great as to bring the plane of the equator to within a very few degrees of that of the ecliptic. Spitzbergen, lying in latitude 80° North, and the sun taking just the same time for its passage from the equator to the tropics, whether the obliquity be great or small, the winter of Spitzbergen would not be very materially shortened, unless the Arctic circle were brought to at least half its present distance from that country. If, however, we suppose this to occur, the obliquity being reduced from its present amount by as much as one-third, it would still leave Spitzbergen far within the Arctic circle ; and its light-receiving position even then would only be about the same which that part of Greenland lying between the 74th and 75th parallels of latitude now occupies. Such a reduction

1 Sir C. Lyell, in his " Principles," states, however, that Sir John Herschel informed him, that although the limit calculated hy Laplace (1° 21') was true as regards the last 100,010 years, yet if millions of years were taken into account, it was conceivable that the variation might be found to extend to three, or even four degrees.

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 09 Aug 2017 at 18:33:27, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800155190 Searles V. Wood, Jun.—The Climate Controversy. 387 in the obliquity, therefore, although it would take those parts of Greenland near Disco, about latitude 70°, in which a similar fossil vegetation is preserved, out of the Arctic circle, and consequently out of the limits within which darkness for any lengthened period prevails, would not remove the difficulties presented by the Spitz- bergen vegetation in any appreciable degree. Two writers, Colonel Drayson and Mr. Thomas Belt, have lately urged that the Glacial period proper was due to an increase in the obliquity of the ecliptic; but Mr. Croll insists that, instead of an increase of cold during the year in high latitudes, or even an increase of winter cold balanced by a corresponding increase of summer heat resulting from an increase of obliquity, the effect of such increase would be to augment the warmth of the polar climate as a whole, by reason that the further the sun receded from the equator, the more heat he would during summer impart to high latitudes at the expense of low ones, while the cold of winter would not be materially aggravated in high latitudes by his greater recession on the opposite side of the equator; and he quotes Mr. Meech's calculations to prove that the poles would, when the admitted obliquity was at its maximum, receive during the year nineteen rays for every eighteen which it receives at the present time. It seems to me, however, notwithstanding this, that there is some fallacy lurk- ing in the contention that the winter cold would not be augmented by an increase of obliquity to the same extent that the summer heat would be. Moreover, if the reasoning which Mr. Croll uses to prove that there is a cumulative tendency in the formation of ice and snow, and consequently of cold, in relation to the Excentricity theory which he advocates, be sound, is it not equally applicable to the Obliquity theory ? and inasmuch, therefore, as the area of the polar circles, or the area in which ice and snow form and remain during winter, would be increased by a greater obliquity (and this in a larger degree than the obliquity itself), would not the effect of that increased area, by the cumulative process, make itself felt in a general refrigeration of climate ? If so, then, by parity of reasoning, a diminution of this area of cumulative cold, by a decrease in the obliquity, should produce amelioration of climate. There will be occasion to recur to this subject of the accumulation of cold in considering the cause No. 3. Taking the view he does, however, as to the effect of an increase in the obliquity, Mr. Croll attributes much influence to it, in combination with the cause to which he refers the greatest influence on climate—the varying excentricity of the earth's orbit; and he contends that, assuming that the astronomers are right in limiting the variation in the obliquity to about 1° 22' on each side of the amount at which it stood at the beginning of the present century, a concurrence of the maximum of this obliquity with the maximum of excentricity would probably cause forest-trees to grow at the pole of that hemisphere which had the sun in perihelion during winter. No. 3.—The cause suggested under this head is the one which, during the past few years, has produced the greatest amount of dis-

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 09 Aug 2017 at 18:33:27, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800155190 388 Searles V. Wood, Jun.— The Climate Controversy. cussion, and therefore demands the most examination. More than ; thirty-five years ago, M. Adhemar advanced a theory that the Noachian deluge was the last of a series of deluges which had occurred during past ages from the melting of snow and ice accumu- ; lated at that pole which lay in the hemisphere where the aphelion, or point of greatest recession of the earth from the sun, occurred during winter. It has long been known that, owing to the pheno- menon called the precession of the equinoxes, the aphelion and perihelion points are transferred from the summer to the winter seasons, and vice versa, every 10,500 years or thereabouts;1 so that for this period the sun is at its greatest distance from the earth during the winter season of one hemisphere and the summer season of the other; while for the next 10,500 years these conditions are reversed. M. Adhemar contended that the effect of this would be, that during the occurrence of the aphelion near the winter solstice of either hemisphere, the polar ice of that hemisphere would be constantly augmenting; and that as the perihelion point returned to the winter season of such hemisphere, this accumulated ice would be dissolved, and great volumes of water be liberated. In 186*5 Mr. Croll revived this theory, in so far as it assumed to account for changes in climate, and added to it the effect which would be produced by the varying excentricity of the earth's orbit, in either increasing or diminishing this refrigerating power of the aphelion ; according as by increased excentricity the aphelion was removed further from the sun, or by diminished excentricity was brought nearer to it. Many objections have been offered to this theory. One was that, whether the excentricity be great or small, an equal quantity of heat is received by the earth during every revolution round the sun ; the augmented distance of the aphelion being balanced by the diminished distance of the perihelion during increased excentricity, and vice versa; so that the augmented heat of the 10.500 summer seasons, due to the occurrence in them of the perihelion, would balance the refrigerating effect of the occurrence during the same number of years of the aphelion in the winter season, and melt all the extra quantity of ice that the increased winter cold had produced; the result being merely that the difference of temperature between winter and summer would be greater or less, according to the position of the aphelion and the degree of excentricity. It seems, however, that this is not precisely true; but that the j quantity of heat received by the earth from the sun in every revo- \ lution round it varies inversely as the minor axis of the orbit; i which would make the quantity of heat received greatest when the \ excentricity was greatest, and vice versa ; thus diminishing any effect produced by an increase of excentricity in augmenting the cold of mid-winter in that hemisphere in which the aphelion occurred at this season. It is also admitted that whatever be the variation thus arising 1 It appears that this period of 10,500 years is subject to large variations ; but it is convenient to speak of it as of that duration.

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 09 Aug 2017 at 18:33:27, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800155190 Searles V. Wood, Jun.—The Climate Controversy. 389 from the length of the minor axis, each hemisphere receives an equal quantity of heat during any one year; the greater contiguity of the sun in perihelion being compensated by the more rapid motion of the earth in its orbit at that time, and vice versd. Mr. Murphy has, since this theory was first advocated by Mr. Croll, endeavoured to show that the occurrence of the aphelion points during the winter season would have the opposite effects to those attributed to it by Mr. Croll; and that it is to the occurrence of the aphelion in the summers of either hemisphere during periods of great excentricity that the glaciation of such hemisphere is to be attributed. Mr. Murphy has lately brought forward another paper, in which he seeks to establish this point; but as it is not yet published, I shall avoid the chance of misrepresenting his views in the mean time by confining myself to merely calling attention to them. Mr. Croll admits that the position of the aphelion point would not, of itself, produce any sensible effect on climate, even with the maximum of possible excentricity, which he gives as making a difference between the sun's distance in aphelion and perihelion of li,212,700 miles; but he contends that when the cumulative tendency of ice and snow and the operation of the great ocean currents are taken into consideration, the true and principal cause of the changes which have occurred in climate has been the combined operation of the position of the aphelion and the varying excentricity of the orbit. As regards this alleged tendency of ice and snow to accumulate at the pole of that hemisphere near the mid-winter of which the aphelion point occurred during periods of a high degree of excentricity, Mr. Croll argues that the increased heat of the sun in summer, due to the corresponding occurrence of the perihelion at that season, would be neutralized by the absorption and reflection by ice and snow of a large part of the heat so received, and by the creation, of fogs which would intercept the rays. With respect to absorption, Mr. Croll argues that 142° Fahr. are absorbed in melting one pound of ice. and completely lost, so far as raising its temperature is concerned, by reason that they are consumed in tearing asunder the molecules against the forces of cohesion which bind them together into the solid form. Is not an equal quantity of heat, however, liberated to raise the temperature of the surrounding air when congelation returns ? With respect to the rays reflected, Mr. Croll does not tell us what becomes of them. They must, it is to be assumed, be absorbed by the atmosphere; but in what way the icy region is prevented from indirectly receiving the benefit of them, when so absorbed, he does not attempt to show. With respect to the fogs, Sir Charles Lyell urged that there were no sufficient data to assume, as Mr. Croll did, that the inten- sity of the sun in a clear sky, which would first melt the ice, would afterwards be sufficiently overcome by fog to check and almost prevent further melting, in spite of the continued supply of exces- sive heat during summer; but Mr. Croll has since collected instances of the prevalence of snow storms and fogs in the Arctic and Ant- arctic regions, and endeavoured to show that it is to these causes

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 09 Aug 2017 at 18:33:27, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800155190 390 Searles V. Wood, Jun.—The Climate Controversy. the snowclad condition of Sandwich Land and the South Georgian Isles—lying in latitudes corresponding, respectively, to the North of Scotland and centre of England—is due. In reference, however, to the interception and absorption of the sun's rays by the creation of fogs, are we not entitled to assume that, since vapours give out in condensation an amount of heat equal to that absorbed in their creation, compensation would be thus afforded, and all the heat intercepted by fogs be eventually restored to the atmosphere of the icy region ? To argue that cold produces ice and snow, and that these again aggravate the cold, and so, by reacting on the cause, tend to produce a continual accumulation of the effect, does to many minds seem arguing in a circle. This Mr. Croll admits, but contends that the argument is none the less sound. He does not, however, pursue his argument to that extent which carries conviction to my mind, but leaves it with an impression that, notwithstanding all that is urged, the agencies to which is attributed a cumulative tendency in ice and snow contain within themselves forces that exactly, but in a less obvious form, compensate for the effects which are attributed to them. From a mean of temperatures taken north and south of the equator,1 it appears that the temperature of the surface of the whole earth is greater when it is in aphelion than when it is in perihelion. This was attributed by Sir Charles Lyell to the heating effect of the great body of land lying in the northern hemisphere—that body of land being exposed to the summer sun while the earth was in aphelion. Mr. Croll, however, regards it as a necessary result of his theory, attributing it to the loss of heat during perihelion, caused by the southern and more glaciated hemisphere being then turned towards the sun, and part of its heat lost by the agencies just discussed. Another and the most important of the causes operating to give influence to the excentricity is, according to Mr. Croll, the gi'eat oceanic circulation, in carrying heat from equatorial to temperate and polar regions; and he has entered into an elaborate examination of the subject to prove that the heat thus transferred is far greater than is generally supposed. He also contends that the position of the aphelion greatly influences the direction of these currents. His argument is too long to summarize; but in the case of the Gulf- stream, for instance, his contention is that with the main configura- tion of the great continents unaltered, this stream, instead of flowing as it now does, would during the glaciation of the northern hemisphere, begin so much further south in mid-Atlantic that, instead of entering the Gulf of Mexico and being thence deflected north-east- wards to Northern Europe, it would be deflected by the coast of South America southwards, as in fact one part of it now is deflected. This alteration Mr. Croll deduces from a previous hypothesis which he seeks 1 I am not aware on what observations these temperatures are based ; but if they were taken at land stations exclusively, or preponderatingly, the fact alleged to result from them may not be free from question.

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 09 Aug 2017 at 18:33:27, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800155190 Searles V. Wood, Jan.—The Climate Controversy. 391 to establish, -viz. that the refrigeration of the northern hemisphere which would arise from the occurrence of the aphelion in its winter, would push the equatorial belt of calms some degrees of latitude further south, and with it the north-east trade, to which he insists the Gulf- stream is due. In reference to this oscillation of the equatorial belt of warmth, Mr. Croll quotes the adoption by Mr. Darwin of the Excen- tricity theory as accounting for the diffusion of certain species of plants, which occur alike in the temperate, but not in the tropical regions of both the northern and southern hemispheres. The view is, that as the cold of the northern hemisphere became greater, some of the more dominant and wide-spreading northern temperate forms invaded the equatorial lowlands, and as the two hemispheres gradually recovered their former temperature, that then, although most of these northern temperate forms so living in equatorial lowlands would have been driven to their former homes or destroyed, some of them would have ascended any adjoining highland, where they would have survived like the Arctic forms on the mountains of Europe. Then, as the southern hemisphere was in turn subjected to a severe glacial period with the northern hemisphere rendered warmer, the southern temperate forms would invade the equatorial lowlands to intermingle with the northern forms so left on the mountains, but from which they now descended. As the warmth again increased, these southerners would return to their former homes, leaving some species on the mountains, and carrying south- ward with them some of the northern temperate forms which had descended from their mountain fastnesses. The necessity for this complex explanation is not apparent, at any rate so far as the continent of America is concerned; as all that would seem necessary for the passage of temperate forms of plants from north to south, and vice versa, is a line of mountains running in that direction of elevation sufficient to induce on their flanks or summits temperate climates in tropical latitudes ; and this, if we make allowance for the proportionate refrigerence of all latitudes during the glacial period, we find on the American continent; since those parts of the chain which now, in Central America, do not from their small altitude furnish this condition of things, would have had at that time temperate climates on their flanks or summits, and the intervals of hot lowland have been so reduced as not to prevent the occasional migration of temperate climate plants from one hemisphere to the other. On the other hand, the common origin of the Greenland blubber whale (Balcena nysticetus), and the right whale of the Antarctic (Balcena australis) at a late period, which has been generally admitted is repugnant to that oscillation of the equatorial warm water belt without curtailment of its breadth, which forms part of the Excentricity theory; and seems to indicate that a curtailment of the breadth of this belt, which now forms so impassable a barrier between these allied species, did actually take place by means of that simultaneous refrigeration of both hemispheres which all the facts hitherto observed in reference to the Glacial period proper appear to me to prove.

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 09 Aug 2017 at 18:33:27, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800155190 392 Scarles V. Wood, Jun.—The Climate Controversy. The theory that the oceanic currents are due to the action of the trade winds is old, but the intimate connexion which Mr. Croll shows to exist between the general oceanic and atmospheric circu- lations is very striking. The influence, however, of these currents, except so far as their direction may be supposed to be altered by the alternate glaciation of the two hemispheres from the position of aphelion, is applicable equally to theory No. 4. To that theory, however, and to the one under consideration, there arises the objection, so far as the Glacial period proper is concerned, that the distance to which the evidences of glaciation extend southwards on the eastern side of North America, beyond what they do in Europe, seem to indicate that the portion of the oceanic circulation to which the existing differences in climate between corresponding latitudes in Europe and North America are due could not have differed much during that period from what obtains now. The theory requiring that there have been many periods of high excentricity, during every one of which there were one or more minor periods of about 10,500 years when the opposite hemispheres were subjected to the alternate conditions of glaciation and warmth, Mr. Croll endeavours to prove by two things. One of these is that glacial periods embracing such minor periods of alternation have occurred at different stages in the Geological record; and the other, that the evidences preserved of that particular period which we are accustomed to call the Glacial afford proof of there having occurred during its progress repeated alternations of climate, or, in other words, interglacial periods of warmth. As to the first of these, he has (as Sir Charles Lyell has, as regards some of the instances, also done in his " Principles ") collected various notices by geologists of formations considered to be indicative of ice action, belonging to Silurian, Devonian, Carboniferous, Jurassic, Cretaceous, Eocene, and Miocene ages respectively; and they no doubt to some readers have a very convincing appearance; but to others they induce more perplexity than conviction. In my own case, not being so familiar with the Palaeozoic and Mesozoic formations as with the Tertiary, I read these instances of Palasozoic and Mesozoic glaciation with respect, though not conviction ; but when I come to the Eocene and Miocene instances, facts clashing entirely with them present themselves. Jn the case of the Eocene, we are offered the evidence of the occurrence, in the unfossiliferous Flysch of the Alps, of a bed containing blocks of all dimensions up to the size of an ordinary church ; and in the Miocene the evidence of a North Italian bed containing blocks both of large dimensions and striated. Now the Eocene formation is complete in England, and is exposed in continuous section along the north coast of the Isle of Wight from its base to its junction with the Oligocene (or Lower Miocene according to some), and along the northern coast of Kent from its base to the Lower Bagshot Sand. It has been intersected by railway and other cuttings in all directions and at all horizons, and pierced by wells innumerable ; while from its strata in England, France, and Belgium, the most extensive collections of organic remains have been made

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 09 Aug 2017 at 18:33:27, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800155190 Searles V. Wood, Jun.—The Climate Controversy. 393 of any formation yet explored, and from nearly all its horizons, for at one place or other in these three countries nearly every horizon may he said to have yielded fossils of some kind. These fossils, however, whether they be the remains of a flora such as that of Sheppey, or of a vertebrate fauna containing the crocodile and alligator, such as is yielded by the beds indicative of terrestrial conditions, or of a molluscan assemblage such as is present in the marine and fluvio-marine beds of the formation, are of unmistakably tropical or subtropical character throughout; and no trace whatever has appeared of the intercalation of a glacial period, much less of successive intercalations indicative of more than one period of 10,500 years glaciation. Nor can it be urged (though this would be the contrary of what is urged in relation to the Glacial period proper, in which the warm intervals are regarded as accompanied by terrestrial conditions, and the cold ones by submergence) that the glacial periods of the Eocene in England were intervals of dry land, and so have left no evidence of their existence behind them, because a large part of the continuous sequence of Eocene deposits in this country consists of alternations of fluviatile, fluvio-marine, and purely marine strata; so that it seems impossible that during the accumulation of the Eocene formation in England a glacial period could have occurred, without its evidences being abundantly apparent. The Oligocene of Northern Germany and Belgium and the Miocene of those countries and of France have also afforded a rich molluscan fauna, which, like that of the Eocene, has as yet presented no indication of the intrusion of anything to interfere with its uniformly subtropical character. It can hardly be contended that the mollusca adapted themselves to a refrigerated sea, and again to a warm one, without the facies of the fauna being changed, because we find during the succeeding period, the Pliocene, the molluscan fauna gradually changing by the disappearance of the tropical and subtropical genera, and by the incoming of species which now generally inhabit Arctic and Boreal seas, thus indicating the gradual approach of those climatic conditions which culminated in the Glacial period. When it is suggested that at two distinct epochs during this uninterrupted prevalence of tropical and subtropical conditions in England and adjoining countries, glaciers descended to the sea several degrees further south, we can scarcely escape the admission that if they did so it could not have been in consequence of any cause involving the existence of the present axis of rotation; for neither lofty land nor ocean currents could have modified the climate of Switzerland or North Italy to such a degree as to allow glaciers to descend to the sea there, while countries lying several degrees of latitude nearer the pole were enjoying a tropical climate. The only thing clear about these so-called Eocene and Miocene glacial beds is that we have much to learn about them ! As to the other fact which is essential to the Excentricity theory, viz. the occurrence during the Glacial period proper of alternations of 10,500 years warm climate, we find no indication of any such

Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 09 Aug 2017 at 18:33:27, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800155190 394 Searles V. Wood, Jun.—The Climate Controversy. alternations during that gradual refrigeration which is indicated by the change in the Pliocene mollusca before referred to ; but this objection Mr. Croll would no doubt meet by his contention that the periods of warmth would be necessarily those of emergence, or, more accurately, of a recession of the sea due to the attraction of the Southern ice-cap; and that it is only the periods of gradually increasing refrigerence (which would be those of submergence in Northern Europe, owing to the transfer of the ice-cap to the northern hemisphere) which the Pliocene mollusca would record. The value of this contention our present certain knowledge of Pliocene land deposits renders it difficult to test by reference to facts, that knowledge being almost confined, in the case of Western Europe, to the Cromer Forest-bed, and the lacustrine formations of the Arno valley; but so far as these cases go, they seem to show no appreciable departure from the kind of climate indicated by the marine deposits of nearly similar age; and afford no support to the idea that they accumulated during intervals of warmer climate. As regards such intervals during the Glacial period itself, Mr. Croll has collected together a number of notices brought forward by geologists, which he considers prove his case. Most of these refer to discoveries in Scotland; but as it appears to me that some of the deposits which are treated by Scotch geologists as Glacial belong to the period which, in reference to English deposits, we are accustomed to call Post-Glacial, during the latter part of which period there occurred, in my opinion, some degree of renewed refrigeration and a partial submergence, it may be convenient to summarize the conclusions to which a study of the English Glacial and Post-Glacial formations has led me; more particularly as Mr. Croll has connected one of the instances upon which he relies to prove these warm intervals of the Glacial period with my name. In the first place, I have long contended that the English Glacial beds, inclusive of the unstratified clays, are of marine origin; and I believe that the gentlemen of the Geological Survey employed in the examination of these beds have mostly come to a similar opinion. We have in England an advantage in possessing the sequence of Glacial formations more complete than they are in Scotland, the earlier members of the series being, according to my view, unrepre- sented in the northern portions of Britain ; and throughout this series, from its commencement, with beds lying in juxtaposition to the Crag, and differing but slightly in their molluscan fauna from the latest members of the Crag series, to its termination in the shelly gravels at high elevations in Lancashire, and at Moel Tryfaen, there is, with one doubtful exception, nothing that I can discover indicative of a cessation either of Glacial climate or of marine conditions. This exception refers to the unconformity that I discover between the Lower and Middle Glacial deposits of East Anglia, which appears to me to be very marked, and to have been connected with the principal excavation of the East Anglian valley-system. Unfortu- nately there is nothing yet found to prove that this valley excavation was the result of a conversion of the Lower Glacial deposits into