fenneman ] THE BOULDEK, COLO., OIL FIELD. 327 
have been employed as are least liable to error when made by the 
driller- Such, for example, are the "showings" of gas and oil, the 
occurrence of water-bearing strata, the character of such water, 
whether fresh or salt, and the hardness of the strata, as revealed by 
the frequency of the change of bits. 
OCCURRENCE OF THE OIL. 
TJie oil-bearing strata. — The beds from which the oil is obtained are 
the highly variable sands or sand rock described above as varying 
between clay shale and silica sand. Such beds may be met with at 
any depth and there is no depth at which such rock is certain to be 
found. Reports might indicate that it is slightly more abundant at 
a depth approaching 2,000 feet, but this may be due to the sharper 
lookout for sand as the well gets deeper. Not all of these strata con- 
tain oil or gas; some of the most porous sands give no indication of 
either. 
The thickness of such beds may be anything up to 100 or 200 feet, 
but a stratum ma}' yield oil or gas from a part of its thickness only. 
Such strata are plainly not so homogeneous as would appear from the 
bailings and are not porous sandstones. Except in an oil well they 
would probably not be called sands at all. It is not uncommon to drill 
many feet into such strata and then strike a showing of oil or gas 
with no attendant change in the texture of the rock that can be 
detected in the bailings. 
The lateral extent of these "sands" is generally small and always 
uncertain. Their outcrops are so few, so short, and so far apart as 
to afford little clue to the continuity of any one bed. As revealed by 
the drill, no one bed can thus far be definitely known to be more 
than half a mile in extent. Even that is a liberal assumption, based 
upon the encountering of sand at similar depths in all the wells 
within a radius of 80 rods. In even the best instance of this kind the 
texture of the sand varies from well to well, some having small quan- 
tities of oil, while others are dry. Moreover, a considerable local dip 
must be assumed in this case in order to correlate the sands of each 
well in a single continuous stratum. Hence it can not yet be affirmed 
with confidence that even in this case there is an uninterrupted sandy 
stratum one-half mile in extent. On the other hand it may well be 
that more discriminating reports would show continuity at many 
places where it does not appear from the reports received. 
The horizontal limitations of these sandy strata are in'obably best 
accounted for upon the supposition that their composition and tex- 
ture vary from place to place, and hence a sand stratum may grade 
into a shale at no great distance. The supposition that the deposits 
are in lenses of somewhat homogeneous character must also be 
admitted as possible, though direct evidence of such stratification 
is lacking. Such lenses should be readily recognized in a group of 
