CHAP. LI 
SUMMARY AND GENERAL DEDUCTIONS 
477 
that, in spite of occasional departures, the normal order remains broadly 
uniform. . , 
With the important exception of the Snowdoman region and possibly 
others, we find that the earlier eruptions of each period were generally most 
basic, and that the later intrusions were most acid. Thus the diabase -lavas 
and tuffs at the base of the Cambrian series of St. David’s are pierced by 
quartz -porphyry veins. The andesites of the Lower Old Led Sandstone 
were succeeded by bosses, sills, and dykes of granite, felsite, and lamprophyre. 
The eruptions of the Carboniferous plateaux began with extremely basic 
lavas, and ended with trachytes, felsites, and quartz -porphyries The basalts 
of the great lava-fields of the Tertiary period are pierced by masses of 
sranophyre and even granite. , . 
There has evidently been, on the whole, a progressive diminution m the 
quantity of bases and a corresponding increase in the proportion ot acid m the 
lavas erupted during the lapse of one volcanic period. This sequence is so 
well marked and so common that it cannot be merely accidental. The acid 
and basic rocks, occurring as they do at each volcanic centre m the same 
relation to each other, are obviously parts of one connected series ot 
eruptions. We seem to see in this sequence an indication of what was 
taking place within the subterranean magma. There was first an extensive 
separation of the more basic constituents, such as the ferro- magnesian 
minerals and ores, and the lavas which came off at that time were heavy 
and basic basalts, and even picrites. The removal of these elements left the 
luacmia more acid, and such rocks as andesites were poured out, until at last 
the” deeper intrusive sills, dykes and bosses became thoroughly acid rocks, 
such as felsite, quartz -porphyry and granite, while if any superficial outflow 
took place it was such a rock as dacite. _ 
In the case of the Tertiary volcanic series there is evidence that after 
the acid protrusions a final uprise of basic material occurred. No satisfactory 
proof of any similar return to basic eruptions has been detected among the 
Palaeozoic formations. But it is possible that some of the basic sills anc 
dykes, the precise age of which cannot be fixed, may really mark such a 
reversion, even in the earlier volcanic periods. 
Some illustrative examples of volcanic cycles from other countries 
were cited in Chapter iii. To these I may add another instance which 
presents a close analogy to some of the phenomena cliar ^er ls tic o 
British examples of Paleozoic as well as of Tertiary age. Monte Venda in 
the Euganean Hills, already alluded to (p. 474), be effed as an inhere - 
in g specimen of an older Tertiary volcano, which has been so dissected by 
denudation as to show not only the succession of its superficial discharge , 
but the position and order of its subterranean intrusions. The volcanic 
eruptions of this neighbourhood, judging from the area which they still cover 
and the height they reach, may have piled up a mountain nva ling 01 
surpassing Etna in dimensions. In Monte Venda the lowest visible igneous 
rocks are sills of oligoclase-trachyte that have been thrust between and have 
highly altered Cretaceous (Tithonian) limestones. Other intrusive sheets 
