CHAP. IV 
VOLCANIC CHRONOLOGY 
47 
igneovis magmas whicli supply volcanic vents thus appear to have been very 
much what they are now from early geological epochs. At least no im- 
portant difference, according to relative age, has yet been satisfactorily 
established among them. 
But although the rocks themselves afford no precise or trustworthy clue 
to their date, yet where they have been intercalated contemporaneously 
among fossiliferous stratified formations, of w’hich the geological horizon can 
be determined from included organic remains, it is easy to assign them 
to their exact place in geological chronology. A determination of this kind 
is only an application of the general principle on which the sequence of the 
geological record is defined. A few illustrations will suffice to make this 
point quite obvious. 
Among the volcanic tufts in the upper part of Snowdon various fossils 
occur, which are identical with those found in the well-lviiown Bala Lime- 
stone. As the accepted reading of such evidence, we conclude that these 
tuffs must therefore be of the same geological age as that limestone. Now 
the position of this seam of rock has been well establisheil as a definite 
horizon in the series of Lower Silurian formations. And we consequently 
without hesitation place the eruptions of the Snowdon volcano on that same 
platform, and speak of them as belonging to the Bala division of the Lower 
Silurian period. 
Again, in West Lothian the tuft's and lavas ejected from many scattei-ed 
pays were interstratified among shales and limestones in which the charac- 
teristic fossils of the Carboniferous Limestone are abundant. There cannot, 
therefore, be any doubt that these eruptions were much younger than those 
of Snowdon, and that they took place at the time when the Carboniferous 
Limestone was being deposited. We thus speak of them as belonging to- 
volcanoes which were active in that early part of the Carboniferous period 
to which the thick Mountain Limestone of Ireland and Derbyshire belongs. 
As yet another illustration of the determination of geological age, an 
example from the plateau-type of eruption may be given. The great basalt- 
plateaux of Antrim and the Inner Hebrides are built up of lavas that lie 
unconformably on the Chalk. They are thus proved to be later than the 
Cretaceous system, and this deduction would hold true even if no organic 
remains w^ere found associated with the volcanic rocks. But here and there, 
intercalated between the basalts, lie layers of shale, limestone and tuff con- 
taining well-preserved remains of plants which are recognizable as older 
Tertiary forms of vegetation. TIris fossil evidence definitely places the date 
of the eruptions in older Tertiary time. 
It i.s clear that, proceeding on this basis of reasoning, we may arrange 
the successive volcanic eruptions of any given district, make out their order 
of se([uence in time, and thus obtain materials for a consecutive history oi 
them. Or, proceeding from that district into other regions, we may compare 
its volcanic phenomena with theirs, determine the relative dates of their 
respective eruptions, and in this way compile a wider history of volcanic 
action in past time. It is on these principles that the general and detailed 
