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Parr I. Secr. i. § 5.] VOLCANIC SUCCESSION. — 265 
_ supported by independent geological testimony. The existence, 
__ however, of large reservoirs of fused rock, at a comparatively small 
depth beneath the surface, may be conceived as probable, apart 
from the effects of crushing. ‘The connection of volcanoes with lines 
of elevation, and consequent weakness in the earth’s crust, is precisely 
what might have been anticipated on the view that the nucleus, 
though practically solid, is at such a temperature and pressure that 
any diminution of the pressure, by corrugation of the crust or other- 
wise, will cause the subjacent portion of the nucleus to melt. 
Along lines of elevation the pressure is relieved, and consequent 
melting may take place. On these lines of weakness and fracture, 
therefore, the conditions for volcanic excitement may be conceived to 
be best developed. Water, able soonest to reach there the intensely 
heated materials underneath the crust, may give rise to volcanic 
explosions. The periodicity of eruptions may thus depend upon the 
length of time required for the storing up of sufficient steam, and on the 
amount of resistance in the crust to be overcome. In some volcanoes 
the intervals of activity, like those of many geysers, return with 
considerable regularity. In other cases, the shattering of the crust, 
or the upwelling of vast masses of lava, or the closing of subterranean 
passages for the descending water, or other causes may vary the 
conditions so much, from time to time, that the eruptions follow each 
other at very unequal periods, and with very discrepant energy. 
Hach great outburst exhausts for a while the vigour of the volcano, 
and an interval is needed for the renewed accumulation of vapour. 
- But beside the mechanism by which volcanic eruptions are 
produced, a further problem is presented by the varieties of materials 
ejected and the differences which these exhibit at neighbouring vents, 
and even sometimes at successive eruptions from the same vent. It 
is common to find that the earlier lavas of a voleano have been acid 
(trachytes, liparites, obsidians, &c.), while the later are basic 
(andesites, basalts, &e.). Richthofen has deduced from observations 
in Europe and North America a general order of volcanic succession 
which has been well sustained by subsequent investigation. He 
states that volcanic rocks may be arranged in five great groups, and 
that all over the world these groups have appeared in the following 
sequence. 1. Propylite; 2. Andesite; 3. Trachyte; 4. Rhyolite; 
5. Basalt... The sequence is seldom or never complete in any one 
locality ; sometimes only one member of the series may be found, but 
when two or more occur they take, it is affirmed, this order, basalt 
being everywhere the latest of the series. Instances have been 
noticed of apparent or real exceptions to Richthofen’s law. But the 
continued study of the great volcanic plateaux of Western America has 
supplied many new examples of its wide application.” 
1 « The Natural System of Voleanic Rocks.” F. Richthofen, California Acad. Sct. 
1868. 
2 See in particular Captain Dutton’s valuable Report on the Geology of the High 
Plateauz of Utah, Washinzton,-1830, p. 64. 
