Notices of New Books. 4375 



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 towards 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 is 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 dependent on the mean annual 

 temperature of the air acting antagonistically 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 sub- 

 stratum at that place. It is not necessary that we should here endea- 

 vour to trace the isothermal line of 32° Fahr., as the reader may ob- 

 tain a correct idea of its general course by consulting Baer's charts. 

 It will suffice to say, that on the continent of America it passes some 

 degrees to the southward of the 60th 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 inclosed in it. By its intervention, entire carcases of the 

 extinct mammoth and tichorhine rhinoceros have been handed down 

 in Arctic Siberia from the drift period to our times, and being exposed 

 by landslips, have revealed most interesting glimpses of the fauna of 

 that remote epoch. Conjecture fails in assigning a chronological date 

 to the time when the drift and boulders were spread extensively over 

 the northern hemisphere : the calculations that have been made of 

 the ages occupied in the formation of subsequent alluvial 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 wilhin the geographical limits of the drift. Whatever 

 may be our speculations concerning the mode in which the carcases 

 in question were inclosed in frozen gravel or mud, their preservation 

 to present times in a fresh condition indicates that the climate was a 

 rigorous one at the epoch of their entombment, and has continued so 

 ever since. Moreover, as large carcases could not, without decompo- 

 sition, be conveyed from a distance by water, it is fair to conclude that 

 the animals lived in the districts in which they are found, or in their 



