150 
The Work of the Geological Survey. 
some districts than to fill in ten square miles in others. It is 
customary in the service to estimate not only the area annually 
surveyed by each officer in square miles, but also the number of 
miles of boundary-line which he has traced. The ratio between 
these two figures affords some measure, though an imperfect one, of 
the comparative complexity or simplicity of the work. In simple 
ground a surveyor need have no difficulty in mapping from 70 to 100 
square miles in a year, each square mile including from 3 to 6 linear 
miles of boundary. But in more rugged and difficult districts it is 
often impossible to accomplish half of that amount of area. In these 
cases, however, the ratio between area and boundary-lines usually 
rises to a high proportion. Thus last year, in Argyllshire, the 
average number of linear miles of boundary-lines was as much in 
one district as 17 miles in every square mile surveyed. 
In mining districts an endeavour is made to express on the maps 
the positions of the outcrops of all seams and lodes, the line of every 
important fault and dyke, with the place of such faults at the 
surface, and where they cut different seams underground. For the 
information necessary to record these data we are mainly indebted 
to the owners and lessees of the mines and pits, who, as a rule, most 
generously give us every assistance. Details, as far as possible, are 
inserted on the 6 -inch Ordnance sheets. Copies are taken of borings 
and pit-sections, and notes are made regarding variations in the 
character of the seams or lodes from one part of a mineral field to 
another. At the same time the district is surveyed in the usual 
way, and by exhausting the surface-evidence the surveyor is not 
infrequently able to supply important additional information beyond 
what can be obtained from the mining-plans. 
It is the necessary fate of all geological maps to become anti- 
quated. For, in the first place, the science is continually advancing, 
and the systems of arrangement of the rocks of the earth’s crust 
are undergoing constant improvement, so that the methods of 
mapping which satisfied all the requirements of science thirty years 
ago are found to be susceptible of modification now. In the second 
place, in the progress of civilisation new openings are continually 
being made in the ground — wells, roads, drains, railways, and build- 
ings are being constructed, whereby fresh light is obtained as to the 
rocks below. Geological lines which were traced with imperfect 
evidence can thus be corrected, and new lines which perhaps were 
not suspected can be inserted. If this kind of obsoleteness over- 
takes geological maps even where only superficial openings are con- 
cerned, much more does it affect those which depict the structure of 
mineral fields still actively worked. The geological maps of Devon, 
Cornwall, and South W ales, made more than half a century ago by 
De la Beche and his associates were for their time admirable in 
conception and excellent in execution. Nothing approaching to 
them in merit had then been produced in any part of the world. 
But the mineral industry of the country has not been standing still 
all these years. Enormous progress has been made in working the 
