380 ANNUAL BEPOKT SMITHSONIAN INSTITUTION, 1910. 



these salt-bearing strata lie so near the beginning of the readable 

 record of life they are singularly instructive, for they give an insight 

 into climatic conditions well back toward the primitive state of 

 things. They challenge at once the view that in these early ages the 

 earth was swaddled in a dense vaporous atmosphere from pole to 

 pole, for under such a vaporous mantle a broad desert tract in India 

 seems scarcely credible. 



If we come forward in time two periods to the deposits of Silurian 

 times, we find great sheets of salt and gypsum underlying the St. 

 Lawrence Basin in New York and westward, with a spread of many 

 thousand square miles. In some parts of the accompanying beds 

 there is complete barrenness of life; in some other parts the life 

 seems to be pauperized, or to be only a remnant selected by hard con- 

 ditions from an ampler fauna. The physical characters of the de- 

 posits of sand, silt, and lime seem to add strength to the interpreta- 

 tion that this great area where now lies a part of our great lake sys- 

 tem was affected in Silurian times by an aridity that gave it scarcely 

 less than a desert aspect. These signal facts join with those of the 

 Salt Range of India in challenging the former picture of a universal 

 envelope of vapor and cloud in all those early times, while it is in 

 keeping with such a diversity of climate as has prevailed in later 

 ages. 



In the next period there are formations that have been interpreted 

 as implying desert conditions, but the evidence is less strong; and 

 we pass on to certain stages of the sub-Carboniferous period next fol- 

 lowing, wherein beds of salt and gypsum are found in Montana, 

 Michigan, Nova Scotia, and Australia, thus implying wide but not 

 general arid conditions. 



Passing on to the Permian and Triassic periods, near the middle 

 of the geologic series, beds of salt and gypsum are found to be phe- 

 nomenally prevalent on both the eastern and western continents, with 

 a surprising range in latitude. The relative paucity as well as the 

 peculiar characteristics of the life of those times seems equally to 

 imply vicissitudes of climate in which aridity was a dominant ele- 

 ment. There seems no tenable way to interpret these remarkable 

 facts of the middle periods except by assuming an even greater prev- 

 alence and intensity of aridity than obtains at the present day. 



So, too, at certain times in later periods, but at certain times only, 

 the stratigraphic record implies atmospheres as arid as those of 

 to-day ; not even^where, indeed, but much as now, in particular areas 

 and at certain horizons. 



These significant facts make up one group of phenomena ; but there 

 is another side to the picture. 



If the record be searched for facts of opposite import, they will 

 come easily to hand. Indeed, as already noted, they seemed to the 



