62 PHYSICAL

GEOGRAPHY.

 

times reach a height of more than two hundred
feet.

The geyser issues from the summit of a conical hillock
of silicious material deposited by the water. A broad,
shallow basin generally surmounts the hillock and forms
the mouth of a deep, funnel-shaped tube. The sides of
both tube and basin are lined with a smooth incrustation
of silica. In the Great Geyser of Iceland, the basin is 52
feet wide and the tube 75 feet deep.

Both the tube and basin are the work of the spring,
being deposited from the silica contained in the highly
heated waters. It is only when the tube has reached a
eertain depth that the spring becomes a true geyser.

hen the depth becomes too great the geyser eruptions
cease, the waters forcing their way through the walls of

the tube to some lower level. Hence, in all geyser re--

gions, numerous deserted geyser-tubes, and simple ther-
mal springs occur.

The waters of some geyser regions are calcareous. In
this case the tube of the geyser is, of course, formed of
limestone.

164, Bunsen’s Theory of Geysers.—Bunsen explains
the cause of geyser eruptions as follows: The heat of the
volcanic strata, through which the geyser-tube extends,
causes the water which fills it to become highly heated.
The water at the bottom of the tube, having to sustain
the pressure of that above it, gradually acquires a tem=
perature far above the boiling-point at the surface. The
temperature of the water in the tube will, therefore, de-
crease from the bottom to the surface.

If now, when the tube is filled, the water, near the mid-
dle, is brought to its boiling temperature, the steam thus
formed momentarily lifts the water iri the upper part of
the tube, when the water in the lower part, released from
its pressure, bursts into steam and forcibly ejects the con-
tents of the tube.

Bunsen succeeded in lowering a thermometer into the
tube of the Great Geyser in Iceland just before an erup-
tion. At the depth of 72 feet he found the temperature
of the water to be 261° Fahr., or 49° above the ordinary
boiling-point.
~ 165. Geyser Regions——There are three exten-
sive geyser regions:

(1.) In Iceland, in the south-western part of
the island, where over one hundred occur in a
limited area.

(2.) In New Zealand, about the centre of the
northern island, where, near the active volcano
Tongariro, over one thousand mud springs, hot
springs, and geysers burst from the ground.

(3.) In Yellowstone National Park, in Wyoming,
where numerous large geysers occur, mostly near
the head-waters of the Madison and Yellowstone
Rivers, at heights often as great as 8000 feet
above the sea-level. Here the boiling-point of

the water at the surface of the geyser, owing to
the diminished atmospheric pressure, is as low
as about 200° Fahr.

A small geyser region is found in California,
near San Francisco.

 

 

166. Nature of the Mineral Substances form-
ing the Reservoir.—The subterranean waters dis-
solve various mineral matters either from the
strata over which they flow, or from their reser-
voirs; this is especially true of thermal springs,
owing to the greater solvent powers of the heated
waters.

The waters of mineral springs generally contain
a number of mineral ingredients: Mineral springs
are divided into various classes according to the
predominating material.

(1.) Caleareous Springs are those whose waters
contain lime in solution.

Thermal waters charged with carbonic acid usually con-

.tain large quantities of lime, which they have dissolved

from subterranean strata. On reaching the surface the
waters cool and part with some of their carbonic acid, and
deposit layer after layer of hard limestone, called travertine.
In this way immense quantities of limestone are brought
to the surface from great depths, leaving huge subterra-
nean caverns.

In portions of Tuscany, Italy, beds of travertine occur
more than 250 feet thick.

(2.) Silicious Springs are those whose waters
contain silicon.

(3.) Sulphurous Waters are those whose waters
contain sulphuretted hydrogen and various metal-
lic sulphides or sulphates.

Sulphurous springs are found in Baden, near Vienna,
and in Virginia.

(4.) Chalybeate Springs are those whose waters
contain iron.

(5.) Salt Springs or Brines are those whose
waters contain common salt.

The springs of Halle, in the Alps of Salzburg, yield
15,000 tons of salt annually. The artesian well of Neu-
salzwerk, Prussia, yields about 28,000 tons annually. In
the United States the springs of Salina and Syracuse are
among the most important. The water in the springs of
Salina is ten times salter than ocean-water. The salt is
obtained from these springs by the evaporation of the
water.

(6.) Acidulous Springs are those whose waters
contain large quantities of carbonic acid gas, as
the Seltzer springs in Germany, and those of
Vichy in France.

167. Petroleum and Bituminous Springs.—Be-
sides the springs above mentioned, there are two
others, closely connected, but which can scarcely
be included in any of the above classes. These
are petroleum and bituminous springs.

Petroleum Springs are those containing rock- or coal-
oil. They rise from large reservoirs containing oil instead

of water. The oil is derived from the slow decomposition,
in the. presence of heat, of various animal and vegetable