
Parrl.Secr.i.§3.)  GEYSERS. 239 
3 from the Great Geyser of Iceland contains in 10,000 parts the 
_ following proportions of ingredients; silica 5-097, sodium car- 
_ bonate 1-939, ammonium carbonate 0:083, sodium sulphate 1-07, 
potassium sulphate 0-475, magnesium sulphate 0-042, sodium 
chloride 2521, sodium sulphide 0-088, carbonic acid 0:557, 
= 11-872. 
As soon as the water reaches the surface and begins both to cool 
and to evaporate it deposits the silica as a sinter on the surfaces over 
which it flows or on which it rests. The deposit naturally takes place 
fastest along the margins of the pools. Hence the curiously fretted 
rims by which these sheets of water are surrounded, and the tubular 
or cylindrical protuberances which rise from the growing domes. 
In course of time the network of underground passages undergoes 
alteration. Orifices that were once active cease to erupt, and even the 
_ water failsto overflow them. Sinter is no longer formed round them, 
and their surfaces, exposed to the weather, crack into fine shaly rubbish 
like comminuted oyster-shells. Or the cylinder of sinter grows up- 
ward until, by the continued deposit of sinter and the failing force of 
_ the geyser, the tube is finally filled up, and then a dry and crumbling 
white pillar is left to mark the site of the extinct geyser. 
§ 3. Structure of Voleanoes. 
We have now to consider the manner in which the various solid 
materials ejected by volcanic action are built up at the surface. This 
inquiry will be restricted here to the phenomena of modern volcanoes, 
including the active and dormant or recently extinct phases. 
Obviously, however, in a modern volcano we ean study only the upper 
and external portions, the deeper and fundamental parts being still 
concealed from view. The interior structure has been in many cases 
laid open among the volcanic products of ancient vents. As these 
belong to the architecture of the terrestrial crust they are described 
in Book IV. The student is therefore requested to take the de- 
‘scriptions there given in connection with the foregoing and present 
sections as related chapters of the study of vulcanism. 
Confining attention at present to modern volcanic action, we 
find that the solid materials emitted from the earth’s interior are 
_ arranged in two distinct types of structure, according as the eruptions 
_ proceed from local orifices or from an extensive system of fissures. In 
the former case volcanic cones are produced; in the latter volcanic 
plateaux or plains. The type of the volcanic cone or ordinary volcano 
is now the most abundant and best known. 
i. Voleanic Cones. 
From some weaker point of a fissure, or from a vent opened 
directly by explosion, voleanic discharges of gases and vapours with 
1 Annal. Chem, und Pharm. 1847, p. 49. 
