52 



NATURE 



[March ,io, 1910 



on the other, the agencies of depletion are efficient and 

 active. There is little escape from the conclusion that, ever 

 since the .birth of air-breathing life, some 30,000,000 or 

 40,000,000 years ago, let us say, the interplav of these 

 agencies of supply .and depletion has been so balanced that 

 neither fatal excess nor fatal deficiency has been permitted 

 to cut short the history of the higher life. 



The dangers of excess or deficiency of the other con- 

 stituents of the air are, indeed, less narrow when named 

 m percentages^ but they are scarcely less real in theoretical 

 possibility. 



The well-being of life is hemmed in between a suitable 

 proportion of moisture in the air dependent on a competent 

 area of water-surface to supply it, on the one hand, and a 

 diluvial excess of water, on the other. Universal deluges 

 and universal deserts would alike be disastrous. A few 

 thousand feet more of water-depth or a few thousand feet 

 less would alike seriously restrict the class of life to which 

 we belong. 



^ In even a more serious way the habitability of the earth 

 IS conditioned on a narrow range of mean temperature — a 

 range, roundly speaking, of 100° Centigrade. This is 

 scarcely <; per cent, of the range of natural temperatures on 

 the earth, and a still smaller per cent. o| the range of 

 temperatures in the heavens. A few miles above us and a 

 few miles below us fatal ■ temperatures prevail. It is 

 profoundly significant that the thermal states of the narrow 

 zone of life on the face of the earth should have been kept 

 within so close variations as to permit the millions of 

 species forming the great genealogical lines leading up 

 from the primitive types to have perpetuated their lineages 

 m unbroken continuity for such ages, while the prevalent 

 temperatures a few miles above them or a few miles below 

 thern, as well as in space generally, would have been fatal. 

 While the necessary heat is dependent on the sun, this 

 control of temperature seems to have been intimately 

 related to the atmosphere, and is a further index of it's 

 specially critical functions. 



To appreciate the full significance of the control of life 

 conditions within these narrow limits when the possibilities 

 were so free and so wide, there is need for some tangible 

 index of the time, but there are at present no means for 

 the close measure of the geologic ages, merely rough 

 estimates of the order of magnitude. Life was far advanced 

 when a readable record first began to be made- but yet 

 since that record began, at least 100,000 feet of sediments— 

 not to choose the largest estimates— have been laid down 

 by the slow methods of wash from the land and lodgment 

 in the basins. The estimate of the years thus represented 

 has been put variously from 50,000,000 to 100,000,000, with, 

 indeed higher figures as well as lower. Merely to scale 

 roughly the order of magnitude, and without pretence of 

 accuracy, let us take the midway figure of ?<;, 000,000 years 

 as representative. Let this Tje divided into fifteen periods of 

 5,000,000 vears each, and these will roughlv represent the 

 technical " periods " of geologists. By this 'rough scale we 

 may space out such of the great events as we need now 

 note. 



_ Slight and changeable excesses of evaporation over pre- 

 cipitation and the reverse prevail widely, but only intense 

 and persistent aridity gives rise to thick deposits of salt, 

 gypsum, and other evaporation products over large areas— 

 with perhaps some exceptions— for in nearly all lar^e 

 natural basins the area that collects rainfall is notably 

 larger than the closed basin within it that alone can retain 

 water for continuous evaporation. It is, therefore, fairly 

 safe to infer clear skies and pronounced aridity when beds 

 of salt and gypsum occur over large areas, especially if 

 accompanied by appropriate physical characters and by s'uch 

 types of life only as tolerate hijth salinity or show pauperisa- 

 tion, or by a total absence of life. 



Now extensive deposits of salt and gypsum are found in 

 the bait Range of India, in strata of the Cambrian periocl. 

 the earliest of the fifteen that make up our rough scale of 

 7J, 000,000 years. Because these lie so near the beginning 

 of the geologic record, they afford a singularly instructive 

 insight into the conditions of the atmosphere well back 

 toward its_ primitive state. They challenge at once the 

 view that in those early ages the earth was swaddled by 

 a dense vaporous atmosphere from pole to pole ; for undpr 

 such a vaporous mantle a great desert tract in India 

 would be scarcely credible. 



NO. 2106, VOL. 83] 



If we come forward in time two periods, to the deposits 

 of the Silurian stage, we find that, underlying the basin of 

 the St. Lawrence in New "^'ork and westward, there stretcii 

 great sheets of salt and gypsum, many thousand square 

 miles in extent. These beds are accompanied by cpmplete 

 barrenness, of life in some parts, by pauperisation of life in 

 other parts, by selections of life according to tolerance of 

 salinity in still other parts, and by harmonious physical 

 characters, all of which combine to add strength to the 

 interpretation. All these imply a degree of aridity approach- 

 ing desert conditions in what is now the well-watered \ 

 region of our Great Lakes. These signal facts join those 

 of the Salt Range of India oi earlier date in challenging 

 the former conception of a universal envelope of vapour and 

 cloud in all those early times. 



In the next period there are formations that have been 

 interpreted as implying desert .conditions, but perhaps on 

 less firm grounds, and we pass on to certain stages in the 

 Sub-Carboniferous period next following, wherein beds of 

 salt and gypsum are found in Montana, Michigan, Nova 

 Scotia, and Australia, which imply like climatic conditions. 

 If we pass on to the Permian and Triassic periods, near 

 the middle of the geologic series, beds of salt and gypsum 

 are phenomenally prevalent on both the eastern and western 

 continents, reaching through surprising ranges of latitude. 

 The relative paucity, as well as the peculiar characteristics 

 of the life of those times, seems equally to imply vicissi- 

 tudes of climate in which scant atmospheric moisture was a 

 dominant feature. There seems no tenable way to interpret 

 these remarkable facts of the middle periods except by 

 assuming an even greater prevalence of aridity than obtains 

 at the present time. So, at times in the later periods, but ■ 

 at times only, the stratigraphic record implies atmospheres 

 as arid as that of to-day, not everywhere, indeed, but in 

 notable areas and in certain horizons. 



These and other significant facts of consonant import 

 form one gfroup of phenomena. 



If, on the other hand, the record be searched for facts 

 of opposite import, they will come easily to hand. Startintj^ 

 near the beginning of the record, it is even more easy to 

 find stages abounding in evidences of prevailing humidity, 

 of great uniformity of climate, and of most congenial life- 

 conditions reaching through widp ranges of latitude. Tf 

 we rested on this selection alone, the old view would he 

 abundantly sustained, but the strata bearing evidences of 

 aridity lie between these. ConiSininc the two sets of facts, 

 the conception seems to force itself uoon us that from 

 the very earliest stages of the distinct life-record onward, 

 there have heen times and places of pronounced aridity 

 much as now, or even more intense, while at other times, 

 intervening between these, more humid and uniform con- 

 ditions prevailed. 



This conception grows in strength as we turn from atmo- 

 spheric states to pievailing temperatures. The body of 

 scientific men have rarely been more reluctant to accept 

 any interpretation of geologic phenomena than that of 

 recent general glaciation on the lowlands of Europe and 

 America in mid-latitudes when that view was first advanced 

 by Louis Agassiz. With the conception of former pervasive 

 warmth then prevalent, it seemed beyond belief that great 

 sheets of ice could have crept over large portions of the 

 habitable parts of Europe and North .'\merica some 

 thousands or tens of thousands of years ago. Belief in this 

 was made easier, however, by the view also then prevalent 

 that the earth had been greatly cooled in the progress of 

 the ages, that the atmosnhere had been much deoleted bv 

 the formation of coal, of carbonates, and of oxides, that 

 the ocean had been reduced by hydration and entrance into 

 the earth, and that thus a stage had been reached that 

 made possible an epoch of depressed temperature and of 

 glaciation. The Ice age, thus theoretically associated, cam^^ 

 to be widely regarded as but the first stage in a series of 

 secular winters destined to le.ad on to the total refrigera- 

 tion of the earth. This view was abetted bv the theory 

 of a cooling sun. The depletingf and the cooling processes 

 were regarded as inevitably prog^ressive. and the final doom 

 of the earth as thus foreshadowed in the near future, 

 geologically speaking. 



But opinion was scarcely more than adjusted to this view 

 when the geolog^ists of Australia, of India, and of Soutb 

 Africa, severally and independently, and later those of. 

 South America, presented evidences of former glaciation 



