HOW TO MAKE A GEOLOGICAL MAP. 
31 
menon produced by pressure subsequent to the consolidation of 
the beds. So difficult is this sometimes, that the late Professor 
Jukes has remarked, “You may sometimes ‘toss up’ which is 
cleavage and bedding’ a ad jointing.” The dip of the beds, the 
direction of which is marked by arrows on the map, is of the 
greatest importance in mapping conformable rocks such as the 
carboniferous, or the secondary strata ; for frequently this and 
the feature together, with some notion of the thickness of the 
beds, will be our sole guides in tracing a boundary for some 
distance. In conformable beds, where the dip is at a low 
angle of 3° or 4°, a very small irregularity in the ground, 
a gentle hollow, may cause the boundary line to run a 
long way from the strike or general line of outcrop of the 
beds along a level surface ; whereas, witlj a high dip, the lower 
bed would run but a short distance even in a deep valley. Care 
must be taken to avoid mistaking false or current bedding 
for the dip. In the tertiary strata, from the general absence of 
hard beds, few dips can be taken ; but they usually maintain 
great regularity and horizontality. In tracing the boundaries 
along escarpments, the ground is often obscured by rain-wash 
and small land-slips. 
The superficial deposits are usually well shown in ditch 
sections, besides the numerous brick-yards or gravel-pits. 
Of course it is impossible here to do more than give a rough 
idea of the nature of the evidence which guides the geologist 
in his survey. The work is not to be learnt from books, but 
can only be gained by experience in the field. The object of 
the Grovernment geological survey is to obtain all the informa- 
tion on the geology of the country. The coal-crops that are 
laid down on the maps are, to a great extent, obtained from 
colliery sections and data. Much information about mining 
has to be gleaned from private individuals ; and the late Pro- 
fessor Jukes tells how he has had to spend many days in search 
of some old fellow who had left the district, but who was said 
to be able to “ tell him all about it.” 
The accompanying Plate (XCIII.), upon which the principal 
classes of stratified rocks are represented, will enable us to point 
out more readily the objects of a geological survey. A glance 
at the index will give the order of succession of the rocks ; and 
it is very important to bear in mind the succession of all the 
British strata, the unconformities, overlaps, and faults which 
affect their arrangement, in order to thoroughly understand the 
phenomena exhibited even in such a small area as this. Thus 
the newer deposits, the alluvium and the gravel, rest irregu- 
larly on the rocks beneath ; the oolites, lias, and trias — rocks 
of Secondary age — are all conformable, and they rest indiffer- 
ently on the upturned edges of the older (Palaeozoic) rocks, 
