
GEOLOGICAL SURVEYING 309 
distance up the slope is not 2” sztu but remanzé, forces his 
instrument at intervals down through the sand, until he 
reaches a place where his borer no longer touches the clay. 
Unless the amount of sand carried down the slope be very 
great, it is obvious that the observer can by such means 
attain a line for his outcrop which cannot be far from the 
truth. 
In low-lying regions of Tertiary deposits, clear natural 
sections are of infrequent occurrence—the best exposures 
being met with along sea-coasts, and in recent artificial 
cuttings and excavations. Frequently, indeed, the geologist 
is largely beholden to the records of deep well-borings, etc., for 
information with regard to the succession of the strata, and 
the probable position of the outcrops. Many hints he will 
doubtless derive from a careful study of the various soils and 
the character of the vegetation, and even from the form of 
the ground. Gravel, for example, being a highly porous 
deposit rapidly absorbs rain, and is, therefore, less liable to 
be washed down and trenched by running water, while 
impervious clay, on the other hand, is readily attacked 
superficially. The former deposit, therefore, will often tend 
to form dry lands with a gentle or more rapidly undulating 
surface. Thick sands, in like manner, will give rise to some- 
what similar dry rolling ground; while clays may form low 
plains or higher tracts trenched in all directions by running 
water. But in countries which, like our own, have been long 
under cultivation, the soils of a Tertiary district are often so 
transformed that it is hard to tell from them what the precise 
nature of the subjacent deposits may be. For the same 
reason, plant-associations in such areas cannot always be 
trusted as guides by the geological surveyor. Such difficulties, 
however, are only likely to happen when the geologist is 
dealing with the outcrops of relatively thin accumulations— 
when, on the other hand, a stratum or series is thick and 
covers wide areas, the nature of the soil and the character of 
the vegetation will help the observer to trace its extent with 
more or less confidence. Speaking in general terms, we may 
say that the mapping of unconsolidated Tertiary deposits is 
carried on in much the same way as the tracing of consoli- 
dated sedimentary strata. Now and again they are gently 

