I50 



NA TURE 



[June 12, 1890 



deposits such conclusions become doubtful, and quite 

 untrustworthy when we are concerned with Palaeozoic 

 times. We must, indeed, admit that as the result of 

 more searching criticism, and the increased knowledge of 

 the facts which the labours of many years have now 

 amassed for us, we are not in a position to answer that 

 most important and fundamental question whether a 

 continuous and universal cooling of terrestrial climate 

 has or has not been progressing from the time of the 

 earliest stratified deposits down to the present day. 



The difificulties with which we have to contend in 

 dealing with this problem may be illustrated by a very 

 significant example. The fact has already been noticed 

 that reef-building corals occur only in warm seas, in 

 which, throughout the year, the surface-temperature 

 never sinks below 20° C. If now we compare the 

 geographical distribution of the reef-building corals of 

 the older formations, we find in very early times, in 

 Silurian and Carboniferous deposits, the remains of such 

 corals beyond the Arctic circle ; at a much later period 

 in the Jurassic formation, we find that they reach only to 

 North Germany and to Southern England ; during the 

 second half of the Cretaceous formation, they do not 

 pass the northern limit of the Alps and the mountains 

 of Southern France, and their northern limit in the first 

 half of the Tertiary era is nearly the same. At the 

 beginning of the second half of the Tertiary era, we 

 find them but scantily represented on the northern bound- 

 ary of the Alps, and abundant only in Southern Europe ; 

 and in the latest subdivisions of the Tertiaries, in Pliocene 

 times, they have almost disappeared from Europe. 



From these facts it might seem almost a manifest con- 

 clusion that there has been a continuous fall of tempera- 

 ture since Silurian times, in consequence of which reef- 

 building corals have retrograded through some fifty 

 degrees of latitude ; nevertheless, on closer examination, 

 we find that such an inference would be altogether pre- 

 mature. In the first place, the Palaeozoic corals differ 

 very essentially from those that now exist, and therefore 

 their requirements in respect of warmth may have been 

 totally different ; further, we have no knowledge of any 

 coral reefs in the far north in all the older formations ; 

 and between the Carboniferous and Jurassic formations 

 which we have cited, there intervene the Permian and 

 the Trias in which we know of no reef-building corals so 

 far north ; the most northerly representatives of the group 

 in Permian times appear in North-Western India, those of 

 Triassic times in the Alps. We are therefore absolutely 

 ig^norant whether these changes of distribution, supposing 

 them to have depended on the temperature, are not to be 

 ascribed to alterations in the distribution of temperature, 

 while there may have been no continuous cooling. Lastly, 

 it is by no means definitely ascertained that the position 

 of the earth's axis has always been the same as at pre- 

 sent ; indeed, there are in the course of geological time 

 certain definite epochs pointing to such a displacement 

 of the poles, of which we have yet to gather the meaning. 

 It may therefore be the case that those parts of the earth 

 at which we find Silurian and Carboniferous corals in the 

 neighbourhood of the pole, were much nearer the equator 

 in those early times than they now are. 



Similar difficulties present themselves in all our attempts 

 to arrive at far-reaching conclusions by this method, and 

 thus we are admonished how great caution must be 

 exercised in the face of so many sources of error. 

 Another method, by means of which it has been sought 

 to attain some holding ground for determining the 

 climatic characteristics of early times, is that, leaving 

 out of consideration the conditions under which nearly 

 related organic types exist at the present time, we should 

 simply regard the extent of the geographic distribution of 

 extinct organisms, and from their wide distribution con- 

 clude the existence of a uniform climate over very great 

 areas, nay even over the entire globe. But in such an 



NO. 1076, VOL. 42] 



attempt the risk of over-estimating the facts is imminent^ 

 and especially is this true in the case of marine organ 

 isms ; in a former state of our knowledge, we mighfwell 

 have believed that ancient forms of life had a wider distri- 

 bution than such as now exist, since our knowledge of the 

 tenants of the present seas related almost exclusively to 

 those of shallow water and the coasts, many of which have 

 a restricted range. But from the epoch-making deep-sea 

 soundings of the last decennia, we have learned much of 

 the inhabitants of the depths of the ocean, and have 

 become aware that they possess much the same characters 

 in all parts of the world ; so that, in this respect, there is 

 no essential difference between the present and former 

 ages. As a fact, we have ascertained, from the distribu- 

 tion of organic life, that climatic zones existed in most of 

 the early periods, and that this has not been done in some 

 cases may be simply ascribed to the fact that they have 

 not yet been rightly investigated. 



Side by side with the diverse indications afforded us by 

 the animal and vegetable worlds, regard must be had to the 

 petrographic characters of the old deposits. We have rocks 

 which have issued from the interior of the earth and have 

 solidified from the fluid state, others have been deposited 

 from water, and in the formation of others, again, ice has 

 played an important part, and this mode of formation is 

 generally recognizable by well-marked characters. For 

 our present purpose, only such masses are important as 

 have been transported by ice and thus brought to their 

 present position, for these alone furnish us with con- 

 clusions as to temperature conditions ; they inform us 

 that, whenever they occur, the cold has been, at least at 

 times, sufficiently great to freeze large masses of water. 



The marks of ice action are well known. A moving 

 glacier polishes and scores or scratches its rock floor, and 

 carries with it fine silt, sand, great and small stones, and 

 even mighty boulders, and deposits these materials in' its 

 moraine, without sifting them according to magnitude, as 

 in the case of transport by water. Polished and grooved 

 rock surfaces, scratched pebbles, and deposition without 

 stratification in a confused mixture of silt, sand, coarse 

 pebbles, and enormous blocks, are the indications of 



I glacier deposits ; in the identification of which, neverthe- 

 less, great caution is necessary. If a glacier reaches the 

 sea or a great lake, under certain conditions, masses of 



I ice may be floated away to great distances, carrying with 

 tbem the enclosed stones and boulders, and often deposit 

 them when thawed out of the iceberg, on the sea-bottom 



I at great distances from their place of origin. Thus the 

 deep-sea investigations of the Challenger show that, in 

 high southern latitudes, in the deep sea and far from any 

 coast, numerous stones lie scattered on the fine silt of the 

 ocean-floor, and these can have reached their present 

 position only by such means of transport. Such indica- 

 tions are, however, not quite undeceptive, since it some- 

 times happens that stones are transported in the roots of 

 trees which are carried by rivers into the sea ; but this 

 kind of transport is operative only to a very small extent. 

 When, however, we find in old formations water-formed 

 deposits of fine clay or sand extending over great areas, 

 in which numerous great stones and boulders are pro- 

 miscuously intermingled, we may infer that they have 

 been transported by floating ice, especially when the 

 stones moreover are scored. 



We have now learned what are the most important 

 indications from which we may draw inferences as to 

 the climatal conditions of past ages, and we have en- 

 deavoured to ascertain how far, and within what limits, 

 such inferences are legitimate. Our next task will 

 naturally be to apply the conclusions thus established to 

 the phenomena which we meet with at different epochs of 

 the past, and to form a conception of the climatic relations 

 of those times, and of the conditions depending on them. 

 It would, however, lead us too far afield were we to discuss 

 each period in detail, and we must restrict ourselves to a 



