CHAPTER IV 
Materials erupted at the Surface— Extrusive Series— cowtmMed. iii. Types of old Volcanoes 
1. The Vesnviaii Type ; 2. The Plateau or Fissure Type ; 3. The Pny Tyire. iv. 
Determination of the I'elative Geological Dates of ancient Volcanoes, v. How the 
Physical Geography associated with ancient Volcanoes is ascertained. 
Having now taken note of the various materials ejected to the surface from 
volcanic orifices, we may pass to the consideration of tliese orifices them- 
selves, with the view of ascertaining under what various conditions volcanic 
action has taken place in the geological past. We have seen that modern 
and not long extinct volcanoes may be grouped into three types, and a 
study of the records of ancient volcanoes shows that the same types may 
be recognized in the eruptions of former ages. The following chapters will 
supply many illustrations of each type from the geological history of the 
Jlritish Isles. In dealing with these illustrations, however, we must ever 
bear in mind the all-powerful influence of denudation. We ought not to 
expect to meet with the original forms of the volcanoes. Some little 
redeetion and experience may be required before we can realize under what 
aspect we may hope to recognize ancient and much-denuded volcanoes. It 
may therefore be of advantage to consider here, in a broad way, which of 
the original characters are most permanent, and should be looked foi as 
mementoes of ancient volcanoes after long ages of denudation. 
iii. TYPES OF OLD VOLCANOES 
The three forms of ancient volcanoes now to be discussed are — 1st, the 
Vesiivian type ; 2nd, the I’lateaii or Fissure type ; and drd, the Fiiy type. 
1. The Vesneian Type . — In this kind of volcano, lavas and fragmental 
ejections are discharged from a central vent, which is gradually built up 
by successive eruptions of these materials. As the cone increases in size, 
liarasitic cones appear on its sides, and in the energy and completeness of 
their phenomena become true volcanoes, almost rivalling their parent 
mountain. Streams of lava descend upon the lower grounds, while showers 
of dust and ashes are spread far and wide over the surrounding country. 
If a transverse section could be made of a modern Vesuvian cone, the 
volcanic pile would be found to consist of alternations of lavas and tuffs, 
thickest at the centre, and thinning away in all directions. At some 
