536 
ELEMENTS OE GEOLOGY. 
CHAPTER XXX. 
AGE or VOLCANIC KOCKs— Continued, 
Volcanic Kocks of the Upper Miocene Period.—Madeira.—Grand Canary.— 
Azores.—Lower Miocene Volcanic Pocks.—Isle of Mull.—Staffa and 
Antrim.—The Eifel.—Upper and Lower Miocene Volcanic Pocks of 
Auvergne.—Hill of Gergovia.—Eocene Volcanic Pocks of Monte Bolca. 
—Trap of Cretaceous Period.—Oolitic Period.—Triassic Period. ^—Permi¬ 
an Period.—Carboniferous Period.—Erect Trees buried in Volcanic Ash 
in the Island of Arran.—Old Ped Sandstone Period.—Silurian Period.— 
Cambrian Period.—Laurentian Volcanic Pocks. 
Volcanic Rocks of the Upper Miocene Period. — Madeira ,— 
The greater part of the volcanic eruptions of Madeira, as we 
have already seen (p. 532), belong to the Pliocene Period, 
but the most ancient of them are of Upper Miocene date, as 
shown by the fossil shells included in the marine tuffs which 
have been upraised at San Vicente, in the northern part of 
the island, to the height of 1300 feet above the level of the 
sea. A similar marine and volcanic formation constitutes 
the fundamental portion of the neighboring island of Porto 
Santo, forty miles distant from Madeira, and is there elevated 
to an equal height, and covered, as in Madeira, with lavas of 
supra-marine origin. 
The largest number of fossils have been collected from 
the tuffs and conglomerates and some beds of limestone in 
the island of Baixo, off the southern extremity of Porto San¬ 
to. They amount in this single locality to more than sixty 
in number, of w^hich about fifty are mollusca, but many of 
these are only casts^ Some of the shells probably lived on 
the spot during the intervals between eruptions, and some 
may have been cast up into the water or air together with 
muddy ejections, and, falling down again, have been depos¬ 
ited on the bottom of the sea. The hollows in some of the 
fragments of vesicular lava of which the breccias and con¬ 
glomerates are composed are partially filled with calc-sinter, 
being thus half converted into amygdaloids. Among the 
fossil shells common to Madeira and Porto Santo, large cones, 
strombs, and cowries are conspicuous among the univalves, 
and Cardium^ Spondylus,, and Lithodomus among the lamel- 
libranchiate bivalves, and among the Echinoderms the large 
Clypeaster called C, altiis, an extinct Eurojiean Miocene fossil. 
