

288 STRUCTURAL AND FIELD GEOLOGY 

continuous exposures of rock are to be met with along the 
seashore and in river-valleys, it nevertheless often happens 
that, owing to the presence of superficial accumulations, the 
rocks in a valley may be concealed for longer or shorter 
distances. But should the observer have previously examined 
the strata over a considerable area, the occurrence of such 
blanks in the evidence does not necessarily disconcert him. 
He probably recognises, in the few sections available for study 
in some particular valley, portions of a series of beds, the 
stratigraphical position of which has already been revealed by 
more continuous sections exposed elsewhere in the same 
district. After he has carefully studied the strata of a wide 
area, he will frequently find that a great thickness of strata 
may show a monotonous alternation of the same kinds of 
rock, say, sandstones and shales, and yet these may exhibit 
sufficient variety of lithological character to allow of the whole 
series being roughly divided. Perhaps thick-bedded coarse- 
grained sandstones and grits with subordinate shales may 
prevail at one horizon, and shales with occasional thin beds of 
fine-grained sandstones may predominate elsewhere. Possibly, 
also, the shales at stated intervals may contain nodules of a 
particular kind, or there may occur at a definite horizon some 
stratum characterised either by its fossils or by certain 
peculiarities of composition, texture, or structure. Beds of 
this kind are not infrequently persistent over considerable 
areas, and when such is the case they are invaluable to the 
field geologist. They may not be of sufficient importance to 
be mapped separately from the series in which they occur, 
but their presence in a section at once indicates the strati- 
graphical horizon. Should the observer have ascertained 
that an “index-bed” of this nature lies at a given distance 
above or below any limestone, coal, or other valuable seam he 
may be desirous of mapping, it is obvious that the appearance 
of the index-bed in a valley must enable him to fix the 
approximate position of the seam he is in quest of—no 
matter how deeply the outcrop of the latter may be buried 
under alluvium. The field-geologist, therefore, cannot be too 
careful in acquiring a full knowledge, not only of the particular 
beds whose outcrops he seeks to trace, but of the varying 
characters of the several groups or series of strata with which 
