Among the largest and best known geysers in the world are those of Iceland, chief among them being the Great Geyser.  Silica is the mineral with which the waters of this fountain are impregnated, and the substance which they deposit, as they slowly evaporate, is named siliceous sinter.  Of this material is composed the mound, six or seven feet high, on which the spring is situated.  On the top of the mound is a large oval basin, about three feet in depth, measuring in its larger diameter about fifty-six, and in its shorter about forty-six feet.  The centre of this basin is occupied by a circular well about ten feet in diameter, and between seventy and eighty feet deep.

Out of the central well springs a jet of boiling water, at intervals of six or seven hours.  When the fountain is at rest, both the basin and the well appear quite empty, and no steam is seen.  But on the approach of the moment for action, the water rises in the well, till it flows over into the basin.  Then loud subterranean explosions are heard, and the ground all round is violently shaken.

Instantly, and with immense force, a steaming jet of boiling water, of the full width of the well, springs up and ascends to a great height in the air.  The top of this large column of water is enveloped in vast clouds of steam, which diffuse themselves through the air, rendering it misty.  These jets succeed each other with great rapidity to the number of sixteen or eighteen, the period of action of the fountain being about five minutes.  The last of the jets generally ascends to the greatest height, usually to about 100, but sometimes to 150 feet; on one occasion it rose to the great height of 212 feet.  Having ejected this great column of water, the action ceases, and the water that had filled the basin sinks down into the well.  There it remains till the time for the next eruption, when the same phenomena are repeated.  It has been found that, by throwing large stones into the well, the period of the eruption may be hastened, while the loudness of the explosions and the violence of the fountain effect are increased, the stones being at the same time ejected with great force.

ERUPTION CAN BE INDUCED BY ARTIFICIAL MEANS

Geysers are found all over the island, presenting various peculiarities.  In the case of one of the smaller ones, which is called Strokr, or the Churn, an eruption can be induced by artificial means.  A barrow-load of sods is thrown into the crater of the geyser, with the effect of causing an eruption.  The sensitiveness of Strokr is due to its peculiar form.  An observer states that, “The bore is eight feet in diameter at the top, and forty-four feet deep.  Below twenty-seven feet it contracts to nineteen inches, so that the turf thrown in completely chokes it.  Steam collects below; a foaming scum covers the surface of the water, and in a quarter of an hour it surges up the pipe.  The fountain