untry is
irrigated, and successfully combines agriculture and mining. There are
ore sampling works and brick-making establishments. Oil and natural gas
abound in the vicinity; there are oil refineries in the city; and in
Boulder county, especially at Nederland, 18 m. south-west, and at
Eldora, about 22 m. south-west of the city, has been obtained since 1900
most of the tungsten mined in the United States; the output in 1907 was
valued at about $520,000. The first settlement near the site of Boulder
was made in the autumn of 1858. Placer gold was discovered on an
affluent of Boulder Creek in January 1859. The town was laid out and
organized in February 1859, and a city charter was secured in 1871 and
another in 1882.

BOULDER (short for "boulder-stone," of uncertain origin; cf. Swed.
_bullersten_, a large stone which causes a noise of rippling water in a
stream, from _bullra_, to make a loud noise), a large stone, weathered
or water-worn; especially a geological term for a large mass of rock
transported to a distance from the formation to which it belongs.
Similarly, in mining, a mass of ore found at a distance from the lode.

BOULDER CLAY, in geology, a deposit of clay, often full of boulders,
which is formed in and beneath glaciers and ice-sheets wherever they are
found, but is in a special sense the typical deposit of the Glacial
Period in northern Europe and America. Boulder clay is variously known
as "till" or "ground moraine" (Ger. _Blocklehme_, _Geschiebsmergel_ or
_Grundmorane_; Fr. _argile a blocaux_, _moraine profonde_; Swed.
_Krosstenslera_). It is usually a stiff, tough clay devoid of
stratification; though some varieties are distinctly laminated.
Occasionally, within the boulder clay, there are irregular lenticular
masses of more or less stratified sand, gravel or loam. As the boulder
clay is the result of the abrasion (direct or indirect) of the older
rocks over which the ice has travelled, it takes its colour from them;
thus, in Britain, over Triassic and Old Red Sandstone areas the clay is
red, over Carboniferous rocks it is often black, over Silurian rock it
may be buff or grey, and where the ice has passed over chalk the clay
may be quite white and chalky (chalky boulder clay). Much boulder clay
is of a bluish-grey colour where unexposed, but it becomes brown upon
being weathered.

The boulders are held within the clay in an irregular manner, and they
vary in size from mere pellets up to