3° 
VOLCANIC PRODUCTS 
BOOK I 
cycles have succeeded each other again and again, at widely separated 
intervals, within the same region. After the completion of a cycle and the 
relapse of volcanic energy into repose, there has been a renewal of the 
previous condition of the subterranean magma, giving lise ultimately to a 
similar succession of erupted materials. 
If we are at a loss to account for the changes in the secpience of lavas 
during a single volcanic cycle, our difficulties are increased when we find 
that in some way the magma is restored each time to somewhat the same 
initial condition. Analogies may be traced lietween the differentiation 
which has taken place within a plutonic intrusive boss or sill and the 
secpience of lavas in volcanic cycles. It can be shown that though the 
original magma that supplied tlie intrusive mass may be supposed to have 
had a fairly uniform composition deep down in its reservoir, differentiation 
set in long before the intrusive mass consolidated, the more basic con- 
stitutents travelling outwards to the margin and leaving the central parts 
more acid. If some such process takes place within a lava-reservoir, it 
may account for a sec[uence of variations in composition. Hut this would 
not meet all the difficulties of the case, nor explain the determining cause 
of the separation of the constituents within tlie reservoir of molten rock, 
whether arising from temperature, specific gravity, or other influence. This 
subject will be further considered in connection with intrusive Bosses. 
Another fact which may be regarded as now well established is the 
persistence of composition and structure in the lavas of all ages. Notwith- 
standing the oft-repeated cycles in the character of the magma, the materials 
erupted to the surface, whether acid or basic, have retained with wonderful 
uniformity the same fundamental characteristics. No part of the contribu- 
tion of British geology to the elucidation of the history of volcanic action is 
of more importance than the evidence which it furnishes for this persistent 
sameness of the subterranean magma. An artificial line has sometimes been 
drawn between the volcanic pi’oducts of Tertiary time and those of earlier 
ages. But a careful study of the eruptive rocks of Britain shows that 
no such line of division is based upon any fundamental differences. 
The lavas of Palaeozoic time have of course been far longer exposed to 
alterations of every kind than those of the Tertiary periods, and certain 
superficial distinctions may be made between them. But when these accidental 
differences are eliminated, we find that the oldest known lavas exhibit the 
same types of structure and composition that are familiar in those of Tertiary 
and recent volcanoes. Many illustrations of this statement will be furnished 
in later chapters. As a particularly strilcing instance, I may cite here the 
most ancient and most modern lavas of the Grand Canon of the Colorado. 
Mr. Walcott and Air. Iddings have shown that in the Lower Cambrian, or 
possibly pre-Cambrian, formations of that great gorge, certain basic lavas 
were contemporaneously interstratified, which, in spite of their vast antiquity, 
are only slightly different from the modern basalts that have been poured 
over the surrounding plateau.^ 
’ \ith Animal Jleport V.S. Ccol. Survey ( 1892 - 93 ). 
