1866. ] Geological Maps. 373 
A geological map of any given district ought to be as nearly as 
possible a faithful representation of the outlines and superficial 
extent of those rock-formations which occur nearest to the surface 
in that area—the vegetable soil being disregarded. But in prac- 
tice we generally find that the geologists disregard, not only the 
vegetable soil, but an important series of “ detrital” or “superficial” 
deposits which overlie and mask the more regularly stratified for- 
mations. It therefore becomes interesting to ascertain why this 
should be the case, and for what reason these so-called “ superficial 
deposits ” should be treated as something differmg in kind from 
the rest of the rock-masses which form the crust of the globe. 
_ From a scientific point of view, it is easy to draw one great 
distinction between the “drift” or “superficial ” deposits and other 
formations—namely, that whereas the latter generally conform to 
certain general rules of dip, strike, &c., and bear a more or less 
definite relation to the strata above and below them, the latter are 
altogether irregular in their occurrence, distribution, and inclina- 
tion. Doubtless exceptions may be quoted which would appear to 
disprove the correctness of even this wide distinction; but we are 
inclined to think that it is, nevertheless, correct in so far as it ex- 
presses a broad general fact. ‘This being the case, it is certainly 
much easier to map the formations older than the glacial period 
than the glacial deposits, and the sands and gravels of more recent 
date. There is also no question that the distribution of the 
regularly stratified formations possesses a much higher scientific 
interest than that of gravel and other drift-deposits, which are of 
more or less uncertain age, and which are chiefly characterized by 
occurring anyhow. 
From an economic point of view it is not quite so easy to pro- 
nounce in favour of one plan to the exclusion of another; but 
we think that the farmers of England have as great a right to 
the consideration of the Director-General of the Geological Survey 
as the miners and the geologists ; in other words, it is expedient 
that the gravels and drifts should be mapped with the same care as 
the older formations, and more especially in districts which do not 
yield valuable minerals. A farmer who knows that some of his 
land has a gravelly subsoil, and the rest a clayey, wishes to ascer- 
tain the boundary-lines of the two deposits. ‘To tell him that the 
clay is London clay, and is only irregularly covered by the post- 
glacial gravel, is to give him information that he neither requires 
nor understands, and to ascertain which he would never spend a 
sixpence. We believe that this fact has of late been recognized by 
the directors of the Geological Survey, and the more recently pub- 
lished sheets of the survey-map of Great Britain contain either 
delineations or indications of the outcrops of the superficial 
deposits. 
