The principal rocks are andesites, but trachytes and basalts are also common.
The rocks are mainly basalts and andesites, together with trachytes and phonolites, and some of the basaltic flows are of enormous extent.
These rocks were followed by the outpouring of the extensive older basalts in the Great Valley of Victoria and on the highlands of eastern Victoria, and also in New South Wales and Queensland.
Its presence contributes to the dark colour of many basalts and other basic rocks, and may cause them to disturb the compass.
They are mostly true felspar basalts, but a few contain nepheline in addition to the felspar.
Ancient schists occur on the east coast south of Angmagssalik, and basalts and schists are found in Scoresby Fjord.
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.
Between this lowland and Armagh city, the early Cainozoic basalts form slightly higher ground, while on the west a strip of Trias appears, overlying Carboniferous Limestone.
Trachytes, rhyolites, andesites and basalts occur, and a definite order of succession has been made out in several areas; but this order is not the same throughout the chain.
The volcanic series include the rhyolite of Nell Island, some obsidian, and the sheets of basalts which form the Cloudy Mountains, Mount Dayman and Mount Trafalgar (an active volcano), and also cover wide areas to the south and west of the Owen Stanley Range.
The basalts of S.
Basalts and Tertiary brown coal deposits enter into the composition of the southern extremity of Bennett Island, and the mountains of Sannikov Land, seen by Toll, have the aspect of basaltic "table mountains."
The later basalts are especially marked by columnar jointing, which determines the famous structures of the Giant's Causeway and the coast near Bengore Head.
Among the Tertiary volcanic rocks those of acid types (granites, granulites) were the first to appear and are developed latitudinally; rocks of intermediate type (dacites, andesites) characterize the Miocene and early Pliocene periods; while the basic rocks (ophites, elaeolite syenites and basalts) attained their maximum in later Pliocene and Quaternary times.
It is found in the basalts of N.
Archean gneiss, Cambrian sandstone, Silurian quartzite, limestone and schist, Jurassic sandstone and limestone, Cretaceous sandstone, and Tertiary basalts, gabbros, and granitic rocks all enter into the composition of the islands.
The two leading types of volcanic areas are the plateaus, in which sheets of porphyrites, basalts and even trachytes were emitted, sometimes with wide discharge of volcanic ashes, and the puys, or isolated vents, or scattered groups of vents, which discharged comparatively a small amount of lava and ashes.
They are prolonged southwards into Antrim, where similar basalts overlying Secondary strata cover a large territory.
In the Isle of Eigg, for example, the basalts had already been deeply eroded by river-action and into the river-course a current of glassy lava (pitch-stone) flowed.
Denudation has continued active ever since, and now, owing to greater hardness and consequent power of resistance, the glassy lava stands up as the prominent and picturesque ridge of the Scuir, while the basalts which formerly rose high above it have been worn down into terraced declivities that slope away from it to the sea.
Recent eruptive rocks, mainly basalts, form a line of hills almost bare of vegetation between Benguella and Mossamedes.
Nepheline basalts and liparites occur at Dombe Grande.
A syenite massif of this age occurs at Mittagong; and leucite has been discovered in Carboniferous basalts by David.
The Cainozoic series of New South Wales contains many interesting volcanic rocks, including leucite-basalts, nepheline-basalts and sodalite-basalts.
Deep leads beneath the basalts occur at Kiandra.
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.
The lower (Ashangi group) consists of basalts and dolerites often amygdaloidal.
Of still more recent date are the basalts and ashes west of Massawa and around Annesley Bay and known as the Aden Volcanic Series.
As the rocks lie in a horizontal position, on most of the islands of the group only the basalts or dolerite are visible.
The basalts are submarine flows which formed the basis of the land upon which grew the vegetation which gave rise to the coals; the effusion of dolerite which covered up the Coal formation was subaerial.
The various decomposing volcanic rocks - tufas, conglomerates and basalts - mingled with decayed vegetable matter, and abundantly watered, form a very fertile soil.
These basalts produce a very rich chocolate soil, and were it not for their influence, the greater part of what is now the most fertile part of the island would have been comparatively poor or altogether sterile.
Lower Cainozoic lacustrine beds with fossil plants, of the same age as those which underlie the older basalts of Victoria, occur in the valleys of northern Tasmania.