474 
SUMMARY AND GENERAL DEDUCTIONS 
BOOK VIII 
have been injected between and have marmorized the Cretaceous strata on 
which Monte Venda stands, among the Euganean Hills. 1 
It is obvious that the time of intrusion of the sills cannot be precisely 
determined. They were not likely to be injected at an epoch when the vol- 
canic magma could find ready egress to the surface. That they did not arise 
before such egress was obtained may be inferred from their petrographical 
characters, which are usually those of the later and not of the earlier out- 
flows of the magma ; and from the fact that they not only lie among the 
rocks below the volcanic series, but intersect the lower parts of f that 
series, sometimes even the higher parts. We may therefore, with every 
probability, regard the sills as among the closing phases of a volcanic 
period. 
As the lavas and tuffs of each volcanic period are intercalated among 
the successive geological formations, a definite beginning and end to the 
period are stratigraphieally fixed. We see exactly where in the sedi- 
mentary series the first showers of ashes fell, and where the last mingled 
with the ordinary sand and mud of the sea-floor. The same record shows 
that the volcanic accumulations were finally washed down, that they sub- 
sided with the rest of the ground around them, and that usually they were 
buried under overlying conformable sedimentary deposits. Thus cones of 
ashes and lava which may have been several thousand feet high completely 
disappeared. 
10. A consideration of the distribution of the volcanic rocks in time 
shows not only how singularly uniform the course of volcanic activity has 
been, but that there is no evidence of the cessation of any of the broader 
petrographical types during geological history. Quite as much variety may 
be observed among the erupted materials of Tertiary time in Britain as 
among those of the early ages, when the earth was younger and its volcanic 
vigour might be supposed to have been greater and more varied than it is 
now. The table on the following page will make these features at once 
apparent. Erom this table it will be seen that while some of the acid 
rocks have not always been extruded, the basic masses have played then- 
part in every volcanic period. 
11. A study of the volcanic products of a long series of eruptions 
within the same geographical region may be expected to throw light on the 
changes that take place during the course of ages in the character of the 
internal molten magma. In a former chapter (vol. i. p. 27) reference was 
ma«e to the subject of volcanic cycles and to the sequence, observed in 
various widely separated parts of the world, among the materials erupted 
rom below. Allusion was likewise made in a later chapter (vol. i. p. 90) to the 
remarkable differences in texture and composition noticeable within some 
ai ° e ^ )0< ^ les ef nptive material, and to the evidence which these differ- 
ent ts furnish of a segregation or differentiation among the constituents of 
G. vom Rath, ZeitseU. JJeutsch. Geol, Gescllsch., xvi. (1864), p. 461. 
Wien., lxxi. (1875), p. 7; Antlitz der Enle, vol. i. p.’ 193. K 
18/7. This volcano is further referred to, posted, p. 477. 
E. Suess, Sitzungsber. 
Iieyer, Die Euganecn, 
