360 Notices of Memoirs — The Geological Survey. 



(h) Sections. — The Vertical Sections are drawn usually on the 

 scale of 40 feet to an inch, and are prepared almost entirely to 

 illustrate the succession of strata in the coalfields. Each slieet 

 generally contains more than one section. The materials for the 

 plotting of these sections are sometimes obtained by actual measure- 

 ments taken by the surveyor himself, but more commonly are 

 supplied by the lessees or managers of the collieries. Sometimes 

 tables of comparative sections are given, in illustration of vai'iations 

 in character and thickness between the seams of coal, ironstone, or 

 limestone in different parts of the same mineral field. 



Occasionally, where a group of strata, though of little industrial 

 importance, possesses great geological interest, a vertical section of 

 it has been constructed and published in the same style as the 

 coalfield sections. In this way sections of the Jurassic rocks in 

 Eastern Yorkshire, of the Lower Lias and Ehsetic rocks in the 

 West of England, of the Tertiary strata in the Isle of Wight, and of 

 the Purbeck group in Dorset have been issued. 



Altogether 90 sheets of Vertical Sections have been published for 

 the three kingdoms. 



The Horizontal Sections have been an important feature in the 

 work of the Geological Survey. De la Beche, recognizing the 

 practical disadvantages arising from the construction of sections 

 without any regard to the proportion between height and distance, 

 instituted the practice of drawing them on a true scale. He adopted 

 the scale of six inches to a mile, and invented a system of patterns 

 for the diiferent kinds of rock, which, as he was himself an artist, 

 are appropriate and effective, for they represent in no small measure 

 the general structure of the rocks. The institution of such sections, 

 in lieu of the distorted diagrams too generally employed, was of 

 great service to the survey itself and also to the progress of geology; 

 for it served to correct the evil influences of distorted drawing, with 

 regard not only to geological structure but to the true forms of the 

 ground. 



As an illustration of the character of these sections and their 

 usefulness in correcting popular misconceptions as to geological 

 structure and the form of the ground, reference may be made 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 textbooks 

 and elsewhere. But in reality both the London Clay and the Chalk 

 are so nearly 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 several hundred feet 

 beneath it in the centre. 



In all, 193 sheets of such sections for the United Kingdom have 

 been issued. 



