44 THE ORDNANCE GEOLOGICAL SURVEY CHAP, n 



out the crops of lodes and mineral seams, as well as of 

 the more important dislocations in the crust of the 

 earth, to note where fossils occurred, and to take such 

 specimens of minerals, rocks, and fossils as might be 

 required for the preparation of the maps for the 

 engraver, and the compilation of material for the sub- 

 sequent explanatory Memoirs. There were likewise 

 levellings to be executed for the purpose of constructing 

 horizontal sections, which were drawn on the scale of 

 six inches to a mile. These sections formed as novel a 

 feature as the detailed maps in the progress of geo- 

 logical surveying. They had been constructed by Logan 

 in Wales, in order to represent accurately the structure 

 of the great South Welsh coal-field. The same scale 

 was adopted by De la Beche, who, with his artistic eye 

 and deft hand, introduced into his horizontal sections 

 a system of representation of geological structure such 

 as had never before been attempted. The sections 

 were on a true scale, vertical as well as horizontal. 

 By carefully chaining and levelling, the topography of 

 the ground was represented correctly, and for the first 

 time the relations between surface-features and under- 

 ground structure were clearly brought out. 



In carrying out the various field-operations of the 

 Survey De la Beche took an active personal interest. 

 He spent the greater part of the year with his officers, 

 and kept himself in touch with the details of their 

 work, besides continuing for some years to carry on 

 independent mapping of his own. As the staff in- 

 creased in number, and Ireland came under his 

 jurisdiction, he was necessarily prevented from doing 

 much himself in actual mapping, and he gradually left 

 more and more to the judgment of his subordinates. 



Being under a military organisation, the surveyors 



