
—— a 

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i 

372 STRUCTURAL AND FIELD GEOLOGY 
size. They also play a most important part in soil-circulation. 
In soils which have long been undisturbed by the plough, 
coarse particles and stones are usually absent. This is 
obviously due in chief measure to the bringing up by the worms 
of fine soil from below, and its deposition at the surface as 
“casts,” which are spread out by the action of rain and wind. 
Eventually, in this way, a more or less considerable stratum of 
fine soil accumulates, and gradually buries any stones that 
may have been lying at the surface. It must be remembered, 
however, that the transport of dust ty wind is also an 
important factor in the formation of fine soil in many regions, 
and that the gradual burial of “ancient monuments” of one 
kind or another is probably, in many cases, largely due to 
the gradual accretion of wind-blown materials. | 
Weathering of Rocks.—We have now enumerated the 
more important epigene agents employed in the formation of 
soils and subsoils. As these several agents are often 
associated in their work, it is sometimes hard, or even 
impossible, to say which has played the dominant rédle in 
certain cases. It is obvious, however, that the disintegration 
of rocks is partly a mechanical, partly a chemical, process, 
and that the ultimate result of superficial action is to break 
up minerals and rocks into soluble and insoluble, or practically 
insoluble ingredients. Even the hardest and most resistant 
of rocks and rock-ingredients must succumb. Those that 
resist solution are eventually reduced by mechanical action 
to finely divided particles, which are readily transported by 
running water or carried on the wings of the wind. Some of 
the harder minerals, and notably quartz, may long survive the 
rock masses of which they once formed a constituent portion, 
and continue to play the same part over and over again. 
Here, for example, is a pebble of liver-coloured quartz picked 
up from a gravelly beach on the Firth of Forth. Whence has 
it come, and what tale has it to tell? Originally it formed a 
portion of some vein or layer traversing the metamorphosed 
rocks of the Scottish Highlands. Detached from its parent 
rock in Old Red Sandstone times, it was rolled down the bed 
of some torrential stream, becoming well rounded in the 
process, until it reached the shore of a great inland sea-— 
the “Lake Caledonia”. of geologists. Together with many 

