158 
The Worh of the Geological Survey. 
district of the country. Each geological formation, as it varies 
from one point to another, is crossed by lines of section, so that by 
comparing these the changes in that formation from district to dis- 
trict can at once be seen. The length of each section varies indefi- 
nitely with the nature of the ground, many of them being upwards 
of 100 miles in length. Thus a series of sections runs from Angle- 
sey and the coast of Merionethshire, across the mountainous ground 
of North Wales, to the plains of the Midlands. Another group 
crosses from the central counties to the South Coast. A connected 
chain of sections traverses the breadth of the island from Liverpool 
to the coast of Yorkshire. 
As an illustration of the character of these sections and their 
usefulness in correcting popular misconceptions as to geological 
structure and form of the ground, I may refer to that which runs 
from Leicestershire to Brighton and passes through London (sheet 
79). What is called the “London Basin” is by many people 
regarded as a deep trough of clay with the Chalk rising steeply 
from under it both to the south and north, and we may see this 
conception embodied in actual diagrams in text-books and else- 
where. But in reality both the London Clay and the Chalk are so 
flat that their inclination can hardly be detected except by careful 
measurement. And the section, accurately plotted from borings 
and well-sections, shows them apparently horizontal, though on 
further inspection we find that their line of junction, which is well 
above the datum-line at either end, lies some way beneath it in the 
centi’e. 
The Horizontal Sections are engraved on copper and published 
in sheets, each of which, if the ground is low, may include six lines, 
or 36 miles of section. The same continuous line of section may 
thus extend over several sheets. Small explanatory pamphlets are 
published with these sheets, giving general information as to their 
formations and their local peculiarities. Each sheet of sections is 
published at the price of 5s. In all 191 sheets of such sections for 
the United Kingdom have been issued. 
Besides the usual Horizontal Sections on the scale of 6 inches to 
a mile, occasional sections on a larger scale are prepared to illustrate 
the geological structure of particular localities. In this way the 
coast-line of Cromer and Yarmouth has been represented in detail, 
and its numerous features of geological interest have been inserted 
so as to exhibit a kind of picture of the arrangement of the strata 
in these changing cliffs. Portions of the coast-line of Dorset and of 
the Isle of Wight have been similarly treated. 
(c) Memoirs. — Obviously, in the course of a geological survey, a 
large amount of detailed information is collected which cannot find 
a place either on the maps or on the sections. This material embraces 
much local detail and a large body of evidence which is of impor- 
tance in general geological inquiry. It can only be properly used 
by being arranged, condensed, and printed. The issue of Memoirs 
of its work has been from the beginning one of the chief occupations 
