LITHOLOGY. 57 



impermeability : a predominance of sandy detritus powderyness, 

 mobility and divisibility. Argillaceous soils are comparatively 

 humid and cold and in dry weather become hardened and form 

 a crust upon their surfaces. Arenaceous soils offer the opposite 

 advantages and disadvantages ; they are often light and sterile 

 and in dry weather soon become at their surfaces arid and 

 parched. 



In the chapter upon Climate we shall see how the ripening 

 of the hay crop and harvest in the low country is in an ordinary 

 season accelerated before the average on the light sandy soils, 

 retarded behind the average on the humid clayey soils ; and 

 how the difference between heavier soils and a somewhat more 

 northern exposure and lighter soils and a more sheltered situa- 

 tion makes at equal altitudes between different parts of our field 

 of study the difference between the Vine, the Fig, and the 

 Spanish Chesnut yielding or not yielding eatable fruit. In 

 Istria M. Tommasini appraises the superiority of the light soils 

 underlaid by limestone over the heavy soils underlaid by 

 argillaceous rock at two degrees of Reaumur's thermometer, 

 even although the former are somewhat hilly and also more 

 northern in position. We shall see also what is the difference 

 in the yield of the Cereal crops between the argillaceous soils of 

 Cleveland and the comparatively porous soils of the Central 

 Valley, and with regard to wild plants we find that there is for 

 them upon the well-marked argillaceous soils a comparatively 

 restricted range of station, and that in tracts of country under- 

 laid by rock of this character and overlaid by detritus in which 

 the argillaceous element preponderates the wild plants which 

 occur are mostly such as are widely distributed throughout 

 Britain and ascend to high latitudes and altitudes ; whilst not 

 unfrccjuently in sandy soils we have pascual and glareal species 

 which are less, abundant and less boreal in their distribution. 

 Under equal climates and at equal elevations, we may say safely 

 that an argillaceous soil has a more humid and a more boreal 

 vegetation ; an arenaceous soil, unless overspread by heath, a 



April 1888. 



