CHAPTER V 
Underground Hiases of Volcanic Action. B. Materials injected or consolidated lieneath tlie 
Surface — Intrusive Series : I. Vents of Eruption — i. Necks of Fragmentary Materials ; 
ii. Necks of Lava-form Materials ; iii. Distribution of Vents in relation to Geological 
Structure-Lines ; iv. Metamorpbism in and around Volcanic Cones, Solfataric Action ; 
V. Inward Dip of Rocks towards Necks ; vi. Influence of contemporaneous Denuda- 
tion upon Volcanic Cones ; vii. Stages in the History of old Volcanic Vents. 
In our profound ignorance of the nature of the earth’s interior, we know as 
yet nothing certain regarding the condition and distribution there of those 
molten materials which form the prime visible source of volcanic energy. 
By the study of volcanoes and their products we learn that the fused sub- 
stances are not everywhere precisely the same and do not remain absolutely 
uniform, even in the same volcanic region. But in what manner and from 
what causes these variations arise is still unknown. We are further aware 
that the molten magma, under a centre of volcanic disturbance, manifests 
from time to time energetic movements which culminate in eruptions at the 
surface. But what may be the exciting cause of these movements, to what 
depth they descend, and over what extent of superficies they .spread, are 
matters regarding which nothing better than conjecture can as yet be 
offered. It is true that, in some cases, a magma of fairly uniform composi- 
tion has been erupted over a vast tract of the earth’s surface, and must 
have had a correspondingly wide extent within the terrestrial crust. Thus 
in the case of the older Tertiary volcanic eruptions of North-Western 
Europe, basalt of practically the same composition was discharged from 
thousands of iissures and vents distributed from the south of Antrim north- 
ward beyond the Inner Hebrides, through the chain of the Faroe Islands 
and over the whole breadth of Iceland. Ij nder the British Isles alone, the 
subterranean reservoirs of molten lavas must have been at least 40,000 square 
miles in united area. If they stretched continuously northwards below the 
Faroe Islands and Iceland, as is highly probable, that is, for 600 miles 
further, their total extent may have been comparable to such a region as 
Scandinavia. 
Was this vast underground body of lava part of a universal liquid mass 
within the globe, or was it rather of the nature of one or more lakes or large 
vesicles witliin the crust ? We can only offer speculation for answer. On 
the other hand, there seems to be good proof that in some districts, both now' 
