THE ORIGIN OF IGNEOUS ROCKS. 
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Uriconian. 
I. Acid lava, breccia, and tuff (felsite and rhyolite). 
II. Basic sills. 
Cambrian. 
Llyn Padarn.—I. Quartz-porphyries, felsites or rhyolites. 
II. Andesite, tuff, and breccia. 
St. David’s.—I. Diabase and felsite (tuffs, mixed). 
II. Olivine-diabases, and porphyrites. 
III. Granite and quartz-porphyry. 
IV. Diabase. 
Silurian 
Arenig. —I. Andesite (tuffs and conglomerate) with quartz- 
andesite, rhyolite, or trachyte becoming more 
acid. 
II. Greenstone, dolerite, (sills), eurite and granophyre. 
III. Olivine-diabase (dikes). 
(Volcanic action possibly continuous from Arenig into Bala.) 
Bala —Lleyn.—I. Pyroxene-andesite. 
II. Felsites (lavas and tuffs becoming more acid.) 
III. Andesite. 
Snowdon.—(Altering lava flows of gray andesite or trachyte 
and felsite.) 
IV. Diabase (sills). 
Y-foel-fras.—I. Felsite. 
II. Agglomerate and tuff (of what?) 
III. Diabases and andesites. 
IV. Granophyre. 
V. Diabase. 
Anglesey.—I. Felsitic tuff. 
II. Dolerite dikes. 
Lake district.—I. Andesites, approaching basalt toward the last. 
II. Felsite (rhyolite toward the top.) 
III. More basic, apparently basalt. 
(Dikes of felsite, quartz-porphyries, diorites, and 
mica trap, (?) minette.) 
Scotland.—I. Porphyrite (lava and agglomerates). 
II. Felsite (tuff). 
III. Gabbro and serpentine (sheets). 
Old Red Sandstone. 
Pentland Hills.—I. Porphyrites and diabases (bedded). 
II. Felsite (filling vent as tuff and breccia). 
III. Felsite, porphyrite, and dolerite (dikes). 
(Also dikes of felsite, minette, lamprophyre, vogesite, and other varie¬ 
ties—that is, acid and very basic rocks.) 
Tuffs of acid material alternate with flows of diabasic and andesitic lavas. 
