

282 STRUCTURAL AND FIELD GEOLOGY 
courses, and elsewhere, and exhausted all the evidence to be 
obtained in railway-cuttings, quarries, and other excavations, 
he will usually find that there are wide areas over which no 
rock appears at the surface. The surface may be concealed 
under thick soils and subsoils, or overspread by superficial 
accumulations of various kinds, as clay, sand, gravel, peat, ete. 
How are such blanks in the evidence to be filled up? How 
can we carry the lines of outcrops across tracts which are 
apparently so hopelessly mantled? Fortunately, it is usually 
possible to follow lines of outcrop even when the rocks them- 
selves are not actually seen, for, although concealed, their 
presence is often betrayed in various ways. The following 
are some of the sources of information of which a keen-eyed 
observer will avail himself :— 
(a) Sozls and Subsoils.—In regions which are not covered 
by glacial deposits (such as boulder-clay), or by thick sheets 
of transported materials (sand, gravel, etc.), the soils will. 
usually indicate the nature of the underlying solid rocks, 
fragments of which are almost certain to occur more or less 
abundantly. These will, of course, be readily detected in 
newly ploughed ground, but when the soil is carpeted with 
vegetation, information is nevertheless often obtainable from 
worm-castings, mole-heaps, rabbit-burrows, etc. A red, sandy 
soil containing angular fragments of red sandstone will indicate 
the presence of red sandstone underneath. Tenaceous clay- 
soils, with few or no stones, will be found to pass downwards 
into marls, clays, or argillaceous shales. Should subangular, 
blunted stones (some of them, perhaps, striated) occur 
numerously in a stiff clay soil, the presence of till or boulder- 
clay is indicated. A soil charged with numerous rounded 
water-worn stones will be found to overlie either a superficial 
deposit of gravel or a decomposed conglomerate. 
In estimating the value of the evidence furnished by surface 
stones, it is well to remember that if the stones, whether sub- 
angular or rounded, should consist of many different kinds of 
rock, they must be derived from an underlying superficial 
accumulation of transported materials, or, as just remarked, 
they may indicate the presence below of a disintegrating 
conglomerate, the outcrop of which the observer will probably 
have already encountered in some natural or artificial ex- 

