146 
IDDINGS. 
represent localizations of the actions during certain parts of 
this period of volcanic activity. 
In the two anniversary addresses of Sir Archibald Geikie, 
which he delivered as president of the Geological Society,* 
an account is given of the history of volcanic activity in 
Great Britain from pre-Cambrian times to the Tertiary—a 
history of the most remarkable series of volcanic events yet 
recorded. The great variability of the rocks in composition 
and the recurrence of similar series of varieties has led Geikie 
to conclude that the rocks of a natural group or of a period 
of volcanic activity have resulted from changes that have 
taken place in the magma during such a period; but in con¬ 
sidering the nature of the changes he states: “ With the im¬ 
portant exception of the Snowdonian region and possibly 
others, we find that the earlier eruptions of each period were 
generally the most basic, and that the later intrusions were 
most acid.” 
Since this order of succession is in conflict with the general 
law of Von Richthofen for Tertiary and recent volcanic erup¬ 
tions, and also with the observations of the present writer, 
and since it is important to learn whether this law is applica¬ 
ble to whole periods of eruption at various geological epochs, 
the writer has taken the liberty to analyze the order of suc¬ 
cession so far as it has been made out in Great Britain and 
has been given in the addresses cited. This has been done 
with the following result, the order of succession being indi¬ 
cated by Roman numerals: 
Lewisian. 
I. Dolerites and basalt. 
II. Peridotites and picrites. 
III. Granite and “ syenite.” 
(Basic rocks, followed by highly basic and highly acid rocks.) 
Dalradian. 
Diorite, epidiorite, hornblende-schist, with basic and 
acid lavas. 
(Intermediate rocks, followed by basic and acid ones.) 
* Geikie (A.) Quart. Journ. Geol. Soc. 8°. London, vol. 47, May 1, 
1891, and vol. 48, May 2, 1892. 
