94 Smithsonian Exploration in Alaska in 1904 



length of time. The depth to which, in northern countries, the sum- 

 mer thaw penetrates, varies with the nature of the soil, but, except 

 in purely sandy and very porous beds, it nowhere exceeds two feet in 

 American or Siberian lands lying within the Arctic Circle. The 

 influence of the sun's rays is not perceptible at this depth until to- 

 wards the close of summer, which occurs at a varying period of from 

 five to ten weeks from the time that the surface of the earth was 

 denuded of snow by the spring thaw. During the rest of the year, 

 even in the forest lands, though not so long there as in the open 

 barren grounds, or tundras, the soil is firmly and continuously bound 

 up in frost. The thickness of the permanently frozen substratum is 

 more or less influenced by its mineral structure, but is primarily de- 

 pendent on the mean annual temperature of the air acting antagonis- 

 tically to the interior heat of the earth. Unless the mean heat of 

 the year in any given locality falls short of the freezing-point, there 

 exists no perennial frozen substratum in that place. It is not neces- 

 sary that we should here endeavor to trace the isothermal line of 32° 

 Farh., as the reader may obtain a correct idea of its general course 

 by consulting Baer's charts. It will suffice to say, that on the con- 

 tinent of America it passes some degrees to the southward of the 

 sixtieth parallel of north latitude, and that while it undulates with 

 the varying elevation of the interior, it has a general rise northwards 

 in its course westerly. 



" Where the permanently frozen subsoil exists it is a perfect ice- 

 cellar, and preserves from destruction the bodies of animals com- 

 pletely enclosed in it. By its intervention entire carcasses of the 

 extinct mammoth and tichorhine rhinoceros have been handed down 

 in arctic Siberia from the drift period to our times, and, being ex- 

 posed by land-slips, have revealed most interesting glimpses of the 

 fauna of that remote epoch. Conjecture fails in assigning a chron- 

 ological date to the time when the drift and bowlders were spread 

 extensively over the northern hemisphere : the calculations that have 

 been made of the ages occupied in the formation of subsequent allu- 

 vial deposits are founded on imperfect data ; and we merely judge 

 from the absence of works of art and of human bones, that the drift 

 era must have been antecedent to the appearance of man upon 

 earth, or at least to his multiplication within the geographical limits 

 of the drift. Whatever may be our speculations concerning the 

 mode in which the carcasses in question were enclosed in frozen 

 gravel or mud, their preservation to present time in a fresh condi- 

 tion indicates that the climate was a rigorous one at the epoch of 

 their entombment and has continued so ever since. Moreover, as 



