

322 STRUCTURAL AND FIELD GEOLOGY 
inferred that outcrops can be always so satisfactorily followed. 
Sometimes, indeed, the ground-rocks are so obscured over 
wide areas, by superjacent glacial or alluvial accumulations, 
that the general geological structure can only be surmised, 
and the following of outcrops becomes impossible. At other 
times the region may be so abundantly faulted that no 
cautious geologist would venture to continue an outcrop 
beyond the point where the rock itself could be proved to 
exist. Occasionally, however, in the case of valuable rocks 
and minerals (coal, ironstone, etc.), the observer, desirous of 
helping the mining engineer in his search for such seams, 
might be justified in indicating by means of interrupted lines 
the general course which he thought the outcrops were likely 
to take. But in doing so, he would be careful to note upon 
the map or in his description that the outcrops were largely 
conjectural, and therefore not to be implicitly trusted. 
Three geological systems are represented on the diagram- 
map—Silurian (S), Carboniferous (C), and Jurassic (J). Each 
of these systems, we shall suppose, has yielded to the 
investigator its assemblage of type-fossils. It will be 
observed, however, that, even in the absence of fossils, the 
geologist could have had no difficulty in detecting the 
presence of three distinct series of strata, and in ascertaining 
the order of their succession. The rock-exposures are so 
numerous that they at once reveal the geological structure. 
The series marked J, for example, rests with a strong uncon- 
formity upon the series C; the latter bearing in like manner 
a similar relation to the series S. Another unconformity of 
less importance occurs in the Carboniferous series, where 
the upper group (C?) is represented as gradually stealing 
across the outcrops of the lower group (C')—the structure, 
indeed, is a combination of overlap and unconformity. The 
accompanying section is taken along the line A—B, and gives 
a view of the general geological structure. 
In his monograph or explanatory memoir of such a 
region, the geologist would probably begin by sketching its 
physical features, after which he would proceed to give some 
account of the general distribution of the several systems, 
and their relation to one another. Next would follow a 
particular description of each, beginning with the oldest. 

