326 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull, an 
tion with the sands of other wells. The composition of these "sands" 
is generally far from that which the name would suggest. They are 
not generally distinguished from the shales in color, but contain vary- 
ing proportions of siliceous grains. They are simply more arenaceous 
beds in the great mass of shale. Individual beds have all possible 
compositions between a good clay and a good silica sand, the latter 
being very rare. The thickness of these sandy beds may reach sev- 
eral hundred feet, but they are generally much thinner. In drilling, 
such beds are distinguished from the shales by their greater hardness, 
by the more rapid wear of the tools, and by the smaller amount of 
suspension in the water of the bailer. The washed-out samples com- 
monly appear under close examination as a collection of gritty shale 
granules whose edges have been rounded under the drill. The better 
grades show an admixture of quartz grains, and exceptional beds are 
almost pure sand. 
"Streaks" and "shells" are other terms used to indicate more than 
usual hardness at certain horizons. In the outcrops of these shales 
may frequently be detected hard beds a few inches or even several 
feet in thickness. They are either very calcareous or stained with 
iron oxide or both, and owe their superior hardness to concentration 
of these substances. Such induration may affect the entire bed 
equally, or may be concentrated into ellipsoidal concretions, which 
may be more or less separated, so that there are all gradations between 
the continuous hard plate and the isolated concretions. The word 
"shell" is suggestive of the latter, as the word "streak" is of the 
former, but in the reports the two words may be taken to be synony- 
mous and alike indefinite. The comminuted fragments of these hard 
masses are not easily suspended in the water of the bailer and (prob- 
ably for this reason, as well as their superior hardness) are not infre- 
quently reported as sand. On the other hand, many of the so-called 
hard streaks are no doubt siliceous. The reports of "lime rock" are 
probably to be traced to the same occurrences. Many concretions, 
also, have been cracked, and the cracks subsequently filled with 
calcite. 
The use of data obtained. — In such a mass of slightly differentiated 
shale the record of a well is somewhat monotonous and few have been 
carefully kept. Moreover, since the distinction between the shale 
and the so-called "sands" is merely one of degree, some drillers report 
large numbers of sand strata where others would report all shale. 
Under such circumstances a complete series of samples would seem 
to be the best possible log book. It has been difficult to obtain these, 
largely because of the same monotony which, to the superficial 
observer, has made the keeping of records unprofitable. Neverthe- 
less, records and samples taken under all possible circumstances have 
been reduced to as much system as possible. For all important cor- 
relations the data have been carefully sifted. Only such observations 
