CHAP. XXXIII 
SEQUENCE OF LAVAS 
1 17 
ot a basic or average and intermediate nature, those of succeeding intervals 
become progressively more acid, but are often found to return again at the 
close to thoroughly basic compounds. 
This law is well illustrated by the volcanic history of Tertiary time in 
Britain. We shall find that the earliest eruptions of which the relative date 
is known consisted generally of basic lavas (dolerites and basalts), but includ- 
ing also more sparingly andesites, trachytes and rhyolites ; that the oldest 
intrusive masses consisted of bosses, sills and dykes of dolerite and gabbro : 
that these intrusions were followed by others of a much more acid character 
" felsites, pitchstones, quartz-porphyries or rhyolites, granophyres and 
granites ; that the latest lava is a somewhat acid rock, being a vitreous 
form of dacite ; and that the most recent volcanic products of all are dykes 
of a thoroughly basic nature, like some of the earlier intruded masses. 
