Volcanoes, like all other mountains, are sub ject to the destructive effects of the atmosphere, rivers, and the sea. In an active volcano the upbuilding by lava flows and frag-mental ejections more than compensates for the loss by weathering, and the cone continues to grow in height and diameter. When the volcano has become extinct, the destructive agencies work unopposed. We find extinct volcanoes in all stages of degradation, from those which look as though their activity might be renewed at any moment, to those which require the careful examination of a skilled geologist to recognize them for what they are.
Fig. 205. - Volcanic neck, Colorado. (U. S. G. S).
In the Pacific States may be found admirable examples of volcanic cones in various stages of erosion. In northern Arizona the picturesque San Francisco mountains, themselves volcanic, are surrounded by numerous small and very perfect cones, hardly affected by weathering (Fig. 21). In northern California stands the noble peak of Mt. Shasta (Fig. 29), which was active till a late geological date and still shows traces of activity in its hot vapours, but has begun to suffer notably from weathering. Still farther north, in the State of Washington, is Mt. Rainier, another volcanic cone, which has been longer exposed to the destructive agencies and has been worn into an exceedingly rugged peak.
Fig. 206. - Diamond mine, showing circular form of volcanic pipe in sandstone; Kimberley, South Africa. (Photograph by Hancox).
These mountains, however, merely exemplify the earliest stages of degradation; as time goes on, the loftiest cones will be worn away, and at last only the worn-down and hardly recognizable stump of the volcano remains, which is known as a volcanic neck. The neck consists of the funnel or vent filled up with the hardened lava of the last eruption, or, less commonly, with a mass of volcanic blocks. Associated with this plug of lava may be preserved the lowest lava flows or tuffs of which the cone was originally built up. If the land upon which the volcanic neck stands be covered by the sea or other body of water, the remnant of the cone will be buried beneath sediments, and a volcanic island may be similarly cut down and covered with sediments. Subsequent upheaval and denudation may at a long subsequent time once more expose the buried cone to view. Several examples of this have been found in Great Britain.
Fig. 207. - Irregularly and columnar-jointed lava flow on sandstone; Island of Staffa, Scotland.
The diamond mines of South Africa are in almost cylindrical pipes, which are cut through stratified rocks and are filled with an irregular agglomerate. On the surface the pipes show no topographical indication of their presence, but are quite level with the ground. The exact nature of these pipes is not well understood; if they are truly volcanic, all traces of the cones and associated ejected masses have been removed by denudation.
Lava Flows and Sheets which were poured out on the surface of the ground may be recognized by the aid of several criteria. In flows of only moderate antiquity, which have suffered little denudation, the nature of the mass may be determined at a glance, and traced to the vent whence it issued. Successive sheets, piled one over the other in a rude bedding, are also evidence that the rocks are surface lavas. Surface sheets may be overlaid by sediments, which were deposited upon a submarine flow, or after depression of the land. Such a flow is then called a contemporaneous or interbedded sheet, and evidently its geological age follows the rule for strata; it is newer than the bed upon which it lies and older than the one which rests upon it.
Fig. 208. - Lava flow on sandstone, Upper Montclair, N.J. (Photograph by van Ingen.) The white line shows the irregular contact.
Fragmental Products (Pyroclastic) are positive proof of volcanic action, for they cannot be formed underground. Coarse masses of agglomerate, blocks, and bombs show that the vent from which they issued was not far away, while beds of fine ashes and tuffs may be made at great distances from their source. All these varieties may be enclosed in true sediments, and may, in part, escape destruction long after the volcano which ejected them has been cut away. The fragmental products are always contemporaneous', and when interstratified with sediments are newer than the underlying, older than the overlying, stratum.
Fig. 209. - Pumice, natural size.