All the volcanic rocks of these islands are submarine stratified tuffs which are penetrated here and there by andesite or diabase dikes.
A superficial layer of recent volcanic tuffs occurs in several parts of the island.
These are overlaid by conglomerates, tuffs, sandstones and arkoses, which perhaps do not all belong to the same period.
Between these two chains are round hills consisting of lavas or sometimes of volcanic tuffs, covered with the long silvery grass which also clothes vast prairies in Java and Sumatra.
The geological sequence of events appears to have been the following: - After the deposition of the Eocene (or Oligocene) limestone - which reposes upon a floor of basalts and trachytes - basalts and basic tuffs were ejected, over which, during a period of very slow depression, orbitoidal limestones of Miocene age - which seem to make up the great mass of the island - were deposited; then elapsed a long period of rest, during which the atoll condition existed and the guano deposit was formed; from then down to the present time there has succeeded a series of sea-level subsidences, resulting in the formation of the terraces and the accummulation of the detritus now seen on the first inland cliff, the old submarine slope of the island.
Above Lima the western chain of the Andes is composed of porphyritic tuffs and massive limestones, while the longitudinal valley of the Oroya is hollowed in carbonaceous sandstones.
From the analogy of the neighbouring countries it is possible that some of the tuffs may be Jurassic, but the other deposits probably belong for the most part to the Cretaceous system.
It chiefly consists of stratified volcanic tuffs rich in coal, lignite, fossilized plants and an invertebrate fauna.
Basic lavas, with andesites, trachytes, tuffs and agglomerates are the most common Scottish rocks of this period.
Porphyrites and tuffs are known in the French Carboniferous.
On the south and south-east the valley is bounded by two volcanoes, Lubuk Raja and Si Buwal Buwali, whence were derived the volcanic tuffs of the valley and of the plateau of Sipirok, with their lakes, which are drained by the Batang Toru and its affluents.
All the rivers of this valley, flowing in deep beds of eroded diluvial tuffs, with a fall as much sometimes as 330 to 660 ft.
In area) filling the hollow of an old volcano, and with rivers which have eroded their beds in the tuffs to a depth of 300 ft.
Here, too, are found fertile tuffs, and the valleys are densely populated.
It is only in the more northerly part of the country that the pipes are filled with blue ground (or " kimberlite "), and that they are diamantiferous; but over a great part of Cape Colony have been discovered what are probably similar pipes filled with agglomerates, breccias and tuffs, and some with basic lavas; one, in particular, in the Riversdale Division near the southern coast, being occupied by a melilite-basalt.
The Lower, with its abundant intercalated lavas and tuffs, extends continuously as a broad belt along the northern margin of the Central Plain, reappears in detached tracts along the southern border, is found again on the south side of the Uplands in Berwickshire and the Cheviot Hills, occupies a tract of Lorne (Oban and the vicinity) in Argyllshire, and on the north side of the Highlands underlies most of the low ground on both sides of the Moray Firth, stretches across Caithness and through nearly the whole of the Orkney Islands, and is prolonged into Shetland.
Farther north nearly the whole of the depression is filled with lavas, tuffs and agglomerates, derived from the Tertiary and recent volcanoes which form the most striking feature of the Andes of Ecuador.
Ormuz series; lavas and tuffs with interbedded cla y s and sandstones.
In Luzon trachytic tuffs are sometimes interstratified with nummulitic limestone, thus showing that the eruptions had already begun in the Eocene period.
On the east side in North Glen Sannox Burn, they are associated with cherts, grits and dark schists with pillowy lavas, tuffs and agglomerates which, on lithological grounds, have been regarded as probably of the same age as the Arenig cherts and volcanic rocks in the south of Scotland.
The whole of this consists of tuffs and lavas, andesites prevailing in the west and rhyolites and dacites in the east.
Igneous rocks are not extensively developed; in Wales they form an important feature and occur in considerable thickness; they are represented by lavas of olivine-diabase and by contemporaneous tuffs which are traversed by later granite and quartz felsite.
The deposits are marine, consisting mainly of sandstone and limestone, together with tuffs and conglomerates of porphyry and porphyrite.
But in the upper Cretaceous and lower Tertiary, especially in north-western Baluchistan, there is an extensive development of volcanic tuffs and conglomerates, which are probably contemporaneous with the Deccan Traps of India.
Very extensive layers of melaphyre and andesite, as also of conglomerates and volcanic tuffs, cover the middle portions of the peninsula.
In Minahassa, at the northern extremity, there is a large area of tuffs and agglomerates consisting chiefly of augite andesite, and in this area there are many recent volcanic cones.
Somewhat later Pliocene deposits in the Val d'Arno, as well as the tuffs associated with the Pliocene volcanoes in central France, yield plants of a more familiar type, a considerable proportion of them still living in the Mediterranean region, though some are only now found at distant localities, and others are extinct.
In the Wenlock beds of the west of the Dingle promontory there are contemporaneous tuffs and lavas.
Contemporaneous volcanic action is recorded by tuffs and lavas south-east of Limerick and north of Philipstown.
The palagonitic breccias, which attain their greatest development in the south of the island and on the tableland, consist of reddish, brown or yellowish rocks, tuffs and breccias, belonging to several different groups or divisions, the youngest of which seems to be of a date subsequent to the Glacial epoch.
Slates, schists, quartzites and limestones form the greater part of the hills, but the Brocken and Victorshohe are masses of intrusive granite, and diabases and diabase tuffs are interstratified with the sedimentary deposits.
We may note the pre-Cambrian lavas and tuffs of the Wrekin district in Shropshire and the somewhat later volcanic rocks of Charnwood; the porphyrites, andesites, tuffs and rhyolites of the Borrowdale volcanic centre, erupted in the Ordovician period, and the Silurian granites of the same region.
The central and most picturesque part of the district is formed of great masses of volcanic ashes and tuffs, with intrusions of basalts and granite, all of Ordovician (Lower Silurian) age.