HOW TO MAKE A GEOLOOICAL MAP. 
29 
they may be traced in places underneath those at a lower level. 
Indeed we find, as a rule, that the older the rocks the more 
wild, rugged, and mountainous is the nature of the ground they 
occupy, for these rocks have of course undergone more indura- 
tion and elevation from heat and pressure than the rocks formed 
subsequently to them ; and they have often been elevated to 
form land for long periods, while newer deposits were forming 
around them, and which, indeed, were made up to a large 
extent from their destruction. Some of our rucked layers of 
cloth should have the summits of their folds cut off to repre- 
sent denudation or wearing away which took place before the 
layers above were deposited. This is called an unconformity, 
an indication of a lapse of time. Then, when we have our 
pile of cloths thus arranged, we may cut imaginary valleys out 
of the even layers that were deposited last ; and then the 
different coloured pieces that are exposed, on looking at the 
surface of the whole mass, would give a very good idea of the 
phenomena exhibited by a geological map. Illustrations of 
this kind in wood have indeed been prepared by Mr. Sopwith, 
and they form admirable models for the student. 
The object of a geological map is therefore to indicate the 
areas where the various rocks of which the earth’s crust is 
formed appear at the surface. And we must bear in mind that, 
although there was a regularity in succession, there was much 
irregularity in the area over which each series was deposited, 
so that some may be absent in places ; and that wherever we find 
one deposit resting on the upturned edges of another, we know 
that between the two there was a great interval of time when 
some other deposits were forming which we shall find elsewhere, 
where no disturbance took place to prevent a regular succes- 
sion. 
Mere soils are omitted in our maps, and in general also those 
irregular superficial deposits of gravel, sand and boulder-clay, 
known as drifts ; but their importance is becoming more and 
more apparent the better they are understood, so that before 
many years they will probably be represented on most of our 
geological maps. 
We may now turn our attention to the way in which our 
geological maps are constructed ; how the lines are drawn which 
separate the different rocks which are exposed at the surface. 
To anyone walking over a flat grass-covered or well-wooded 
country, probably the difficulties attending a minute survey 
would appear insurmountable, and they might conclude that a 
great deal is done by conjecture. It is true that the geologist 
is in a great measure forced to be guided by inference when 
there is no direct evidence ; but when he has surveyed all round 
any obscure tract of country, the inferences which guide him in 
