472 
SUMMARY AND GENERAL DEDUCTIONS 
BOOK VIII 
and s° far as we yet know, the whole of the Mesozoic periods within the area 
ot Britain were absolutely unbroken by a single volcanic eruption. The 
c nonological value of this enormous interval of quiescence may, perhaps, 
never be ascertainable, but the interval must assuredly cover a large part 
o geological time. It was an era of geological calm, during which the 
Iriassic, Jurassic and Cretaceous formations were slowly accumulated over 
the larger part of Europe. The stratigraphical quietude was not indeed 
unbroken. The widespread subsidence of the sea-bottom was interrupted 
here and there by important upheavals, and considerable geographical 
c langes were m process of time accomplished. But, save in one or two 
widely separated areas of Europe, there were no active volcanoes over the 
whole continent. 1 Here again the scarcity or absence of intercalated 
volcanic rocks is in harmony with the general stratigraphy of the 
formations. 
9. After the prodigious interval represented by the whole of the 
Mesozoic and the earlier part of the Tertiary formations, a time of dis- 
turbance arose once more, and the great basalt-floods of the north-west 
were poured forth. Evidence has been adduced in the foregoing chapters 
that this latest volcanic period was one of vast duration; that it was 
marked by long intervals of quiescence, and by repeated renewals of 
volcanic energy. Yet over the area of Britain the whole of its manifesta- 
tions were probably comprised within the earlier (Oligocene and perhaps 
eai y locene) part of older Tertiary time. Since its eruptions ceased, 
another interval of profound quiescence has succeeded, which still con- 
tinues. But this interval is almost certainly of less duration than that 
which elapsed between the Palaeozoic and Tertiary outbursts. In other 
vords, remote as the date of these Tertiary volcanoes appears to be from 
our own day, it comes much nearer to us than did the era of the last 
1 ernuan eruptions to the earliest of the Tertiary series. 
,, 10 ; the dissecfcion which prolonged denudation has effected among 
the old volcanic centres of Britain, materials are supplied for studying the 
sequence of events from the beginning to the end of a volcanic period. 
These events have generally followed the same tolerably well-defined 
order. 
In the case of fissure-eruptions, rents formed in the crust of the earth 
and communicating with the surface have allowed lava to rise and flow out 
above ground, either from the lips of the fissures or from vents opened along 
the lines of chasm. The thousands of parallel dykes in Britain remain as 
evidence ot this mode of the ascent of the molten magma. Lines of laro-e 
cones of the Vesuvian type may be presumed to have risen along guiding 
fissures in the terrestrial crust. 
But it is evident from a study of the British examples that the exist- 
ence of a fissure in the visible part of the crust is not always necessary for 
t le production of a volcanic vent. In hundreds of instances, communication 
* The Triassi0 eruptions of Predazzo and Monzoni were important, and 
said to occur in the Cretaceous system in Portugal and Silesia. 
traces of others are 
