
GEOLOGICAL MAPS AND SECTIONS 323 
The several faults and the eruptive rocks would likewise call 
for ample notice. In short, every detail of scientific interest 
and economic importance would be duly set forth in its 
proper place. But if his monograph were written for experts 
only, the geologist would necessarily leave much unsaid, 
knowing that his readers might be relied upon to fill in out- 
lines and to draw obvious inferences for themselves. He might, 
indeed, not infrequently content himself with a bare narration 
of facts, and leave these and his map to tell their own tale. 
Interpretations and explanations would only be called for 
in cases where the evidence was incomplete or not quite 
clear. 
The beginner, however, who essays to work out the 
geological structure of a district, would do well to ponder 
over the evidence as it grows, and endeavour to realise the 
particular conditions under which the various rocks and 
rock-structures originated. His object is not only to make 
a correct geological map and to present a detailed report of 
what he has observed, but to picture to himself as clearly as 
he can the succession of changes which have taken place in 
the region surveyed. If he is dealing with sedimentary strata, 
he must be on the alert to notice every variation in the 
character of the deposits, every fact that would seem to throw 
light upon the conditions that obtained at the time the strata 
were being accumulated. As the evidence furnished by 
fossils is always most important, he will be on the constant 
outlook for these. Only by keeping each kind of evidence— 
that of the fossils and that of the rocks themselves—con- 
stantly in view, can we hope to read geological history aright. 
If we have previously made ourselves well acquainted with 
the nature and mode of formation of sediments now being 
laid down in lakes, estuaries, and seas, and have acquired a 
sufficient knowledge of the various ways in which organic 
remains come to be entombed, we shall be prepared to give 
a good account of any series of sedimentary strata we may 
encounter. Most of the fossils we may detect will in all 
likelihood belong to well-known genera—probably most of 
the species themselves will already have been recognised by 
palzontologists—so that with the combined evidence of rocks 
and fossils the observer will be in a position to realise the 
