CHAP. LI 
SUMMARY AND GENERAL DEDUCTIONS 
47i 
manifestations since the early beginnings of geological time. The composition 
and structure of the materials erupted from the , ..terror 
hut little change. The manner in which these materials have been chs 
charged has likewise persisted from the remotest periods. I he three modern 
t™ of Vesuviau ernes, puys and Sssure-eruptions ean be seen to have 
nlaved their parts in the past as they do to-day. ... 
1 Among the earliest igneous masses of which the relative geological date 
can be fixed are the dykes which form so Striking a system among the 
Archaean rocks of the north-west, and show how far back the modem type of 
volcanic fissures and dykes can be traced. No relic, indeed, has survived of 
any lavas that may have flowed out from these ancient fissures, but so far as 
regards underground structure, the type is essentially the same as that of the 
Tert £ cones of lava and tuff comparable 
to thot of s y ucV vents as Vesuvius and Etna. In the Lake the 
pile of material ejected during Lower Silurian time was at least 80 
9000 feet thick. In the Old Led Sandstone basins of Central Scotland 
there were m 0re than one mass of lavas and tuffs thicker than those of 
A 6S ThlTpuys of the later half of Palaeozoic time closely resembled then- 
Tertiary successors in Central France, the Eifel, and the Phlegnean hie is. 
Nor as regards extent and vigour, did the eruptions of the geological 
past differ in any important respect from those of the present tune.. There 
f B assuredly no evidence that volcanic energy has gradually waned since the 
dawn of geological history. The latest eruptions of North-Western Europe, 
forming the Tertiary basalt-plateaux, far exceeded m area and possibly a so 
in bulk of material discharged, all the eruptions that mid preceded them ir 
the ? °^-^ while the Xertiar y eruptions showed no diminution of 
vigour it is undoubtedly true that the volcanic energy has not manifested 
itself in a uniform way since the beginning of geological time There have 
been periods of maxLum activity followed by others of lessened force 
Thus if we take a broad view of the general features of volcarnc act 
during the Paheozoic ages in Britain, we see clear evidence of a rad 
diminution in its vigour. The widespread outpourings of lav . ' 
the Silurian period in England, Wales, Scotland and Ireland 
by the somewhat diminished, though still important, eruptions of 
CM Bed Sandstone basins. The latter were followed bj « 
whole prolonged volcanic succession ending m the small scattered rents 
^riwan Viod. There were of course oscillations of relative enew 
1 r bN historv some of the maxima and minima being of considerable 
moment. But though progress towards extinction was not regular am 
uniform it was a dominant feature of the phenomena. . ’ . . 
8 i be limi... voice*®, were the lee. of the long raheosme ser.ee, 
