202 THE EVOLUTION OF DERBYSHIRE SCENERY. 



freezes. In freezing it expands, and the cohesion between 

 the particles being overcome, they are thrust asunder, and when 

 a thaw occurs the rock crumbles. This may be noticed in a 

 ploughed field after a frost, or upon wet sand. Sometimes 

 during, or shortly after a frost, long fibres of ice may be seen 

 projecting from such a sand, some of them bearing upon their 

 ends pebbles or fragments of earth. Hence the tendency is for a 

 soft sandstone to crumble into sand. Limestones (excepting 

 oolites and magnesian), however, usually absorb but little water, 

 and so the amount of this action is exceedingly small. What 

 happens in the case of limestone is that water contained in 

 the cracks of the rock expands in freezing, and rends it into 

 blocks and fragments. One effect of the solvent action of water 

 upon the surface of limestone is to bring out their fossiliferous 

 character. The crystalline structure in the fossils causes them to 

 resist more successfully the weathering to which the more or less 

 impure calcareous matrix yields, and this gives rise to the beauti- 

 ful weathered surfaces from which stand crinoids, corals, and 

 shells in relief An interesting instance occurs in the carboniferous 

 limestone of Durham and Northumberland. The face of the 

 unaltered rock shows no signs of certain little grains which make 

 their appearance as the rock weathers. As decomposition pro- 

 ceeds, a disintegrated mass is formed, consisting of a small 

 foraminifer — Saccammina Carteri. 



The effect of a landscape depends largely upon the nature and 

 amount of the vegetation present, and the close connection 

 between geology and vegetable growth is frequently overlooked. 

 The physical and chemical character of the soil depends upon the 

 underlying rock, from which it has mainly been derived ; thus we 

 get sandy, clayey, or calcareous soils, and they determine to a 

 very large extent the kind of tree or grain which shall thrive upon 

 them. One soil frequently lacks something which another 

 possesses, so that often at a junction between a limestone 

 and sandstone, or clay, where the products of decomposition mix 

 together, we get a fertile strip which indicates — too often in vain — 

 the kind of treatment necessary to render the adjacent land 



