478 
SUMMARY AND GENERAL DEDUCTIONS 
BOOK VIII 
ot trachyte follow in the overlying Cretaceous strata (Neocomian and 
Scaglia). It is not until the older Tertiary formations are reached that 
undoubted tuffs and lavas occur, indicative of truly interstratified volcanic 
materials. These formations, consisting of nummulitic limestones and other 
strata together with fossiliferous tuffs, show that the volcano began as a 
submarine vent. It discharged dark basic dolerites and tuffs. The highest 
lava, however, crowning the summit of the mountain is a trachyte. There 
appears to have been a rapid decrease of the bases in the magma, for the 
later lavas were rhyolites, accompanied with rhyolitic tuffs of Oligocene age, 
and followed in the eud by the black vitreous trachyte of Monte Sieva. 
12. From the evidence detailed in these volumes, it appears that the 
sequence from basic to acid discharges was on the whole characteristic of 
each eruptive period. It is obvious, however, that as the protrusions of 
successive periods took place within the same limited geographical area, the 
internal magma during the interval between two such periods must in some 
way have been renewed as regards its constitution, for when, after long 
quiescence, eruptions began once more, basic lavas appeared first and were 
eventually followed by acid kinds. This cycle of transformation is admirably 
exhibited in Central Scotland, where the andesites of the Old lied Sandstone 
with their felsite sills are followed by the limburgites, picrites and other highly 
basic lavas at the bottom of the Carboniferous plateaux, succeeded in turn 
by the andesites, trachytes and acid sills of that series. When the puy 
eruptions ensued, the magma had once more become decidedly basic. 
That the true explanation of these alterations is of a complex order may 
be inferred from the exceptions which occur to the general rule. I have 
alluded to the Snowdon region, where the acid rhyolites are followed by more 
basic andesites, and where the sills are also more basic than the superficial 
lftvas. In the Arenig and Cader Idris country the sills are likewise more 
basic than the bedded lavas. Among the Carboniferous puys of the basin of 
the Firth of Forth, the sills are not sensibly more acid than many of the 
superficial basalts, and they even include such rocks as picrite. Possibly in 
this last-named region we see an arrested sequence, the volcanic protrusions 
having from some cause ceased before the general uprise of the more acid 
magma. 
