Correspondence — 7^. M. Deeley. 95 



any time between about 30 and 40 years of age. The evidence 

 afforded by the parasagittal ridge on the left parietal, by the 

 meningeal grooves, and by the positions occupied by the fragments 

 of the lambdoid suture upon tlie occipital and parietal fragments, 

 corroborates the correctness of this identification of the median plane. 



coE-i^EsiPoisriDEiisrcE:. 



THE FREEZING OF SURFACE SOIL. 



Sir, — In Nature for 1911 I descril)ed tlie occurrence of tooth-like 

 projections of fibrous ice which formed on exposed pieces of chalk partly 

 buried in the soil. Here the water had not frozen in the chalk but 

 on the surface, fresh supplies of water coming from below through 

 the capillaries of the stone to freeze at the surface. Each pore would 

 seem to have produced a separate crystal fibre, the fibres from each 

 pore adhering to one another and resembling fibrous gypsum in 

 appearance. Occasionally when the supply of water from below 

 varied over the chalk surface, strains were set up by the irregular 

 growth of the ' ice-teeth ', which caused the mass to split along the 

 fibres and grow in spiral forms. The amount of ice produced was 

 several times greater than the volume of the chalk upon which it 

 formed, showing that the water had passed from the wet ground 

 through the chalk and frozen on the surface. 



Since describing the above I have seen a "Note on the action of 

 Frost on Soil ", by Milner Eoberts, in the Journal of Geology^ vol. xi, 

 pp. 314-17. He says, "The action of frost in altering the surface 

 of the soil was well shown during the pei'iod of cold weather which 

 prevailed over the Puget Sound district from February 10 to 18, 1903. 



"The first night's frost liad its usual effect of raising the surface 

 of loose ground, which was well illustrated by gravelly soil. A layer 

 of ice consisting of vertical prisms five-eighths of an inch long formed 

 during the night at a depth of about five-eighths of an inch below^ the 

 surface, thus raising the overlying material without otlierwise 

 disturbing it. The cold of the following night produced a similar 

 layer of ice almost an inch thick below the first one, raising the latter 

 along with its load of sand and gravel. 



" The following conditions prevailed and seem to have controlled 

 the formation of these many storied forms: (1) ground which was 

 not frozen and which was readily permeable to moisture ; (2) freezing 

 temperature at night; (3) mild thawing in the daytime; and (4) 

 considerable moisture in the soil." 



During the present week similar conditions have prevailed here, 

 and similar effects have been produced on a gravel walk. In the Figure, 

 p. 96, which is full size, a and h are two layers of fibrous ice resting 

 upon wet clayey soil d. During the first night's frost the pebbles 

 and some soil were lifted. On the following night the layer of ice 

 h was formed. It, however, raised with it a thin layer of soil c. 

 Particles of soil were scattered throughout the ice, liaving been 

 detached from the soil below and lifted by the growing ice fibres. 



