393 
(Geological Survey of England and Wales. 
Hurting plans of salt-workings and placed their details in the six-inch 
maps, and has further collected tables of the levels of the brines at 
various periods, reducing these levels to ordnance datum, and thus 
showing the height of the upper and lower rock-salt surfaces. 
Carboniferous . — It is in the re-examination of the great coal- 
field of South Wales that the chief recent operations of the Survey 
in the Carboniferous system have lain. Sufficient progress has now 
been made to show of how much practical value a detailed survey of 
this coalfield will prove to be. Mr. Strahan, who has had charge 
of this work, soon ascertained that, while the great thickness and 
uniformity of character of the widespread “ Pennant Grit ” makes it 
difficult to obtain indications of the geological structure over large 
tracts of ground, the position of a certain coal-seam known as the 
“ Mynyddislwyn vein ” affords an excellent horizon from which the 
lie of the other strata can be followed in great detail. He has 
accordingly devoted special attention to tracing the outcrop of this 
seam and the trend of the numerous faults which have been met 
with in working it. He has had occasion to examine a large series 
of plans of old workings, and to reduce from these the necessary 
data upon the six-inch ordnance maps. When these maps are 
completed, with all the available detailed information, they will 
probably afford a sufficient and accurate guide to the depth and dip 
of the various coal-seams over a large part of the area. The infor- 
mation thus worked out, combined with a precise geological mapping 
of the ground, will prevent the waste of large sums of money in 
seeking for coal, by showing exactly the limits within which the 
seams may be looked for and the depths at which they may be 
expected. 
Devonian . — The maps of Devon and Cornwall were the first on 
which the Geological Survey began its operations. The region which 
they represent, besides the importance of its mineral industries, is 
one of great geological complication, which could not be properly 
worked out on maps of so small a scale as one inch to a mile and so 
inaccurate in their topography. Moreover, at the time when these 
maps were made, geological science was far from being so well 
equipped as it now is for attacking such problems as are presented 
by the rocks of the South-west of England. It has long been 
recognised, therefore, that a total re-survey of that region was needed ; 
but the state of progress of the survey of other parts of the country 
has hitherto prevented this work from being undertaken on an 
adequate scale. But as the eventual re- survey, which must sooner 
or later be carried out, will be greatly facilitated by an accurate 
determination of the stratigraphical horizons of the Devonian rocks, 
and a detailed mapping of these in some one district, Mr. Ussher 
has been employed in conducting these operations in the south of 
Devonshire. By a sedulous scrutiny of the ground he has been 
enabled to detect the presence of organic remains previously 
unnoticed, and by their aid to distinguish and trace the three great 
divisions of the Devonian system over the district between Newton 
Abbott and Plymouth. 
