174 GEAVEL AND PLACER MINING IN ALASKA. [bull. 263. 
values to be. The disparities were in part accounted for by attribu- 
ting them to spilling, leakage, faulty discharge of buckets, incomplete 
excavation b}^ apparatus, incomplete disintegration during washing in 
screening, and to losses in the gold-saving apparatus. It seemed, 
however, as if these losses should be only about 10 per cent, and the 
other 40 per cent was explained by the well-known general statement 
that "prospecting results are always high." 
Up to the time that considerable areas previously prospected with 
numerous drill holes were dredged, the results of drill prospecting 
were believed to be absolutely reliable, making the allowance, how- 
ever, of about 50 per cent for values to be recovered by the dredge. 
After these areas had been worked, however, the dredge managers 
found much to surprise them, for where ground had prospected well 
it had dredged poorly, and vice versa. It is asserted that in these 
instances both the drilling and dredging were so conducted that no 
great reliability could be attached to the results, but this explanation 
is far from satisfactory. 
In the use of the drilling machine to sample alluvion, as in all other 
prospecting, unusual care must be exercised to obtain results that 
shall give a correct idea of the values and of the other characteristics 
of the ground under examination. It is unquestionably true that 
careless sampling has been the cause of many failures in dredging 
operations. The seeming simplicity of this prospecting method has, 
as in other apparently simple determinations, resulted in crude manip- 
ulation and misleading results. In some instances there has been a 
misleading affectation of accuracy in tabulating results of drill pros- 
pecting. Careless location and distribution of drill holes have given 
faulty results. Nevertheless, the percussion drilling machine is peer- 
less for testing most areas that may be available for exploitation by 
the dredging method. 
The drilling machines are used for several purposes. It is necessary 
to determine whether the material is too hard to make dredging 
profitable, and for this purpose drilling machines are not entirely 
satisfactory. In several areas prospected ))y drilling machines and 
thought suitable for dredges the material proved to be so hard that a 
machine different from the one designed was required for successful 
exploitation. Jf the prospecting had been done with more care this 
unfavorable condition would have been learned. 
Reliable results call for the determination of the presence and distri- 
bution in the vertical section of any considerable content of tenacious 
clay and large bowlders, and here again the drill does not yield entirely 
satisfactory results. The depth of material must be ascertained and, 
where the bed rock is a so-called "false," the greatest care is neces- 
sary in order to determine its position. The character, hardness, and 
roughness of the bed rock must be learned, and the drill unfortunately 
