148 
IDDINGS. 
Carboniferous. 
(The plateaux in general precede the puys.) 
(A) Plateaux. —I. Chiefly porphyrites (variable composition), more 
basic rocks intercalated near the bottom, the 
ultra basic possibly intrusive (later). 
II. More acid lavas (sanidine trachyte), preceded by 
some basic flows. 
Garlton Hills.—I. Basic lavas (olivine-basalt, and (?) picrite) and 
porphyrite (andesite). 
II. Trachyte, with occasional flows of andesite. 
(B) Puys. —I. Basalts mostly (lavas) (some with olivine, some 
without). 
II. Porphyrite occasionally (lavas), (with intrusive 
sheets, necks, and dikes of same kinds of rock). 
Tertiary. 
I. Basalts, with some trachytic tuffs interbedded, 
cut by gabbros. 
II. Trachytes, felsites, quartz-porphyries, granophyre, 
and granite. 
III. Basalt. 
From the foregoing crude analysis it appears that there 
are frequent alternations of more silicious and less silicions 
lavas, and that with few exceptions the series are closed with 
basic lavas, usually in dikes or sills. Moreover, the first 
eruptions are seldom the most basic, but are more frequently 
rocks of intermediate composition. This is also the case in 
certain districts of Tertiary rocks, according to Judd,* who 
has shown that in the Western isles of Scotland there occurs 
beneath the basalts an older accumulation of lavas, consist¬ 
ing of altered andesites, together with “ rhyolites, dacites, 
and sanidine-trachytes, intercalated with which are a few 
basalts.” The history of -volcanic action throughout the 
geologically vast range of time in Great Britain, though not 
yet completely deciphered, appears to accord with what has 
been observed in many regions of Tertiary and recent activ¬ 
ity. The order of eruption indicates a variation in most cases 
from intermediate to more and more extreme varieties of 
* Judd (J. W.) Quart. Journ. Geol. Soc. 8°. London, 1874, vol. 30, p. 
236, and ibid., 1890, vol. 46, pp. 342-385. 
