316 ' Notices of Memoirs — The Geological Survey. 



filling the bottom of the glens, the sheets of flow-peat and the 

 ragged peaty mantle that hangs down from the cols and smoother 

 ridges, the recent alluvia and the successive stream -terraces, the 

 lines of raised beach and the estuarine silts — all these and more 

 must be noted as the surveyor moves along, and must be duly 

 chronicled on his map and among his notes. 



It is obvious that the progress of the mapping in such ground 

 cannot be rapid. Tf the work is worth doing at all, it should be 

 well done, and if well done, it must be done slowly and carefully. 

 It is evident also that the total area surveyed in a year, if given 

 in square miles, affords no guidance whatever as to the amount 

 of labour involved. There may be a hundredfold more exertion, 

 physical and mental, required to complete a single square mile 

 in some districts than to fill in twenty square miles in others. It 

 is customary in the Survey to estimate not only the area annually 

 mapped 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 mountainous 

 and difficult districts it may be 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, 

 in the Scottish Highlands the average number of linear miles of 

 boundarj'-liues sometimes rises to as much 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. The 

 information necessary to record these data is mainly furnished by 

 the owners and lessees of the mines and pits, who, as a rule, most 

 generously give the Survey every assistance. Details as far as 

 possible are inserted on the six-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 civilization, new openings are continually 

 being made in the ground : wells, roads, drains, railways, and 



