q 
Part If. § vi} FRAGMENTAL ROCKS—PSAMMITIC. 159 
+ 
Subsoil.—The broken-up part of the rocks immediately under 
the soil. (See Fig. 92.) Its character of course is determined by 
that of the rock out of which it is formed by subaerial disintegration. 
Blown Sand.—Loose sand usually arranged in lines of dunes, 
fronting a sandy beach or in the arid interior of a continent. It is 
piled up by the driving action of wind (Book III. Part II. Section i.). 
It varies in composition, being sometimes entirely siliceous, as upon 
shores where siliceous rocks are exposed; sometimes calcareous, 
where derived from triturated shells, nullipores, or other calcareous 
organisms. Layers of finer and coarser particles often alternate, as 
in water-formed sandstone. On many coast-lines in Hurope grasses 
and other plants bind the surface of the shifting sand. These layers 
of vegetation are apt to be covered by fresh encroachments of the 
loose material, and then by their decay to give rise to dark peaty 
seams in the sand. Calcareous blown sand is compacted into hard 
stone by the action of rain-water, which alternately dissolves a little 
of the lime and re-deposits it on evaporation as a thin crust cementing 
the grains of sand together. In the Bahamas and Bermudas, 
extensive masses of calcareous blown sand have been cemented in 
this way into solid stone, which weathers into picturesque crags and 
caves like a limestone of older geological date.* 
Gravel, shingle.— Names applied to the coarser kinds of rounded 
waterworn detritus. In gravel the average size of the component 
pebbles ranges from that of a small pea up to about that of a 
walnut, though of course many included fragments will be observed 
which exceed these limits. In shingle the stones are coarser, ranging 
up to blocks as big as a man’s head or larger. These names are 
applied quite irrespective of the composition of the fragments, 
which yaries greatly from point to point. As a rule the stones 
consist of hard crystalline rocks, since these are best fitted to with- 
stand the powerful grinding action to which they are exposed. 
River-sand, Sea-sand.— When the rounded water-worn detritus 
is finer than that to which the term gravel would be applied it is 
ealled sand, though there is obviously no line to be drawn between 
the two kinds of deposit, which necessarily graduate into each other. 
The particles of sand range down to such minute forms as can only 
be distinctly discerned with a microscope. The smaller forms are 
generally less well rounded than those of greater dimensions, no 
doubt because their diminutive size allows them to remain suspended 
in agitated water, and thus to escape the mutual attrition to which 
the larger and heavier grains are exposed upon the bottom (Book III. 
_ Part Il. Section ii.). So far as experience has yet gone, there is no 
method by which inorganic sea-sand can be distinguished from that 
of rivers or lakes. As a rule, sand consists largely (often wholly) of 
quartz-grains. The presence of fragments of marine shells will 
of course betray its salt-water origin; but in the trituration to which 
1 For interesting accounts of the Molian deposits of the Bahamas and Bermudas, 
see Nelson, Q. J. Geol. Soc. ix. p. 200, and Sir Wyville Thomson’s “ Atlantic,” vol. i. 
