Architectural variety and solidity are favoured in the buildings of the city by a wealth of beautiful building stones of varied colours (limestones, sandstones, lavas, granites and marbles), in addition to which bricks and Roman tiles are employed.
The first eruptions piled up huge domes of lavas rich in soda, including the geburite-dacites and sOlvsbergites of Mount Macedon in Victoria, and the kenyte and tephrite domes of Dunedin, in New Zealand.
Lavas dip in all directions from the central crystalline core, pointing to the conclusion that the main portion of the mountain represents a single volcanic mass.
In the southern region of unfolded beds are found the lavas of the " harras " of Arabia, and in India the extensive flows of the Deccan Trap. In the central folded belt lie the great volcanoes, now mostly extinct, of Asia Minor, Armenia, Persia and Baluchistan.
The chlorite-schists are often of igneous derivation, such as ash-beds or fine lavas which have been metamorphosed.
Among these lavas is the "pipe" amygdaloid of which many blocks have been transported great distances down the Vaal river.
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.
Rhyolitic lavas frequently are more or less vitreous, and when the glassy matter greatly predominates and the; crystals are few and inconspicuous the rock becomes an obsidian; the chemical composition is essentially the same as that of granite; the difference in the physical condition of the two rocks is due to the fact that one consolidated at the surface, rapidly and under low pressures, while the other cooled slowly at great depths and under such pressures that the escape of the steam and other gases it contained was greatly impeded.
Basaltic lavas are reported to have been found in the Aldan region.
Geology.'--The Eastern Cordillera., which, however, is but little known, appears to consist, as in Bolivia, chiefly of Palaeozoic rocks; the western ranges of the Andes are formed of Mesozoic beds, together with recent volcanic lavas and ashes; and the lower hills near the coast are composed of granite, syenite and other crystalline rocks, sometimes accompanied by limestones and sandstones, which are probably of Lower Cretaceous age, and often covered by marine Tertiary deposits.
The lavas and ashes ejected by these volcanoes consist of liparite, dacite, andesite and basalt.
These rocks, which include some highly siliceous lavas, form part of the Eocene series that is so conspicuously displayed above Carlingford in Co.
In Rossel Island (Roua or Arova) occur crystalline schistose and volcanic rocks, and in Misima (St Aignan) limestones and lavas in addition.
Kerguelen Island is of undoubted volcanic origin, the prevailing rock being basaltic lavas, intersected occasionally by dikes, and an active volcano and hot springs are said to exist in the south-west of the island.
The Cainozoic volcanic history of New Zealand begins in the Oligocene, when the high volcanic domes of Dunedin and Banks Peninsula were built up. The Dunedin lavas including tephrites and kenytes correspond to the dacite eruptions in the volcanic history of Victoria.
The building up of these domes of lavas of intermediate chemical type was followed by the eruption of sheets of andesites and rhyolites in the Thames English Miles so Ioo 200 Cretaceous 'a '
Of boulder beds and amygdaloidal lavas belonging to the Vaal River System.
Bauxite, probably derived from the decay of lavas, is found between Glenarm and Broughshane, associated with brown and red pisolitic iron-ores; both these materials are worked commercially.
But by far the most important of the Tertiary rocks are the volcanic lavas, agglomerates and ashes, which cover so much of the country.
The basic lavas are usually darker and denser than lavas of acid type, and when fused they tend to flow to great distances, and may thus form far-spreading sheets, whilst the acid lavas, being more viscous, rapidly consolidate after extrusion.
Thus the uplifted, dislocated and dissected lava sheets of the Yellowstone National Park in the Rocky Mountains on the east (about the headwaters of the Snake river) are associated with the older lavas,of the Columbian plains.
In the north, where dislocations have invaded the field of the horizontal Columbian lavas, as in south-eastern Oregon and north-eastern California, the blocks are monoclinal in structure as well as in attitude; here the amount of dissection is relatively moderate, for some of the fault faces are described as ravined but not yet deeply dissected; hence these dislocations appear to be of recent date.
The Cascade Range is in essence a maturely dissected highland, composed in part of upwarped Colombian lavas, in part of older rocks, and crowned with several dissected volcanoes, of which the chief are (beginning in the north) Mts Baker (Io,827 ft.), Rainier (14,363 ft.), Adams (12,470 ft.) and Hood (11,225 ft.); the first three in \Vashington, the last in northern Oregon- These bear snowfields and glaciers; while the dissected highlands, with ridges of very irregular arrangement, are everywhere sculptured in a fashion that strongly suggests the work of numerous local Pleistocene glaciers as an important supplement to preglacial erosion.
The region was by no means a peneplain before its slanting uplift; its surface then was hilly and in the south mountainous; in its central and still more in its northern part it was overspread with lavas which flowed westward along the broad open valleys from many vents in the eastern part: near the northern end of the range, eruptions have continued in the present cycle, forming many cones and young lava flows.
These beds, with intercalated lavas, form the mountainous west shore of Lough Mask, the east, like that of Lough Corrib, being formed of low Carboniferous Limestone ground.
Mauna Kea is not nearly so old as the Kohala Mountains, but there is no record of its eruption, nor have its lavas a modern aspect.
Hitchcock, Hawaii and its Volcanoes (Honolulu, 1909); Augustin Kramer, Hawaii, Ostmikronesien and Samoa (Stuttgart, 1906); Sharp, Fauna (London, 1899); Walter Maxwell, Lavas and Soils of the Hawaiian Islands (Honolulu, 1898); W.
Eruptive rocks of supposed Cretaceous age are met with in these outer islands, but Tertiary and recent volcanic lavas are confined to the innermost arc. Halmahera lies outside these arcs.
Basic lavas, with andesites, trachytes, tuffs and agglomerates are the most common Scottish rocks of this period.
The present volcanoes lie along a line (with offshoots) which runs parallel to the west coast, but some distance to the east of the fissures from which the early Tertiary lavas were poured.
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 axis along which they have been elevated runs north-east and south-west, and on either flank a series of " green rocks " appears, consisting of altered amygdaloidal andesitic lavas, intrusive dolerites, coarse gabbros and diorites, and at Beagh-beg and Creggan in central Tyrone ancient rhyolitic tuffs.
Contemporaneous volcanic action is recorded by tuffs and lavas south-east of Limerick and north of Philipstown.
These are overlain, perhaps unconformably, by a great thickness of lavas and volcanic breccias (Pniel volcanic series, Beer Vley and Zeekoe Baard amygdaloids), and these in turn by the quartzites, grits and shales of the Black Reef series.
The Ongeluk volcanic series, consisting of lavas and breccias, conformably overlies the Griquatown series; while the grits, quartzites and conglomerates of the Matsap series rest on them with a great discordance.
In Ecuador the depression between the Eastern and Western Cordilleras is almost entirely filled with modern lavas and agglomerates; in Colombia the corresponding Cauca depression is almost free from such deposits.
In some cases these sedimentary rocks lie deeply buried under lavas poured out by volcanoes long extinct.
The cones themselves are composed largely of acid andesites, but many of the lavas are augite andesites and basalts.
There seem to have been two periods of eruption, and as some of the lavas have flowed over Quaternary gravels, the latest outbursts must have been of very recent date.
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.
Specular iron ore occurs in the form of brilliant metallic scales on many lavas, as at Vesuvius and Etna, in the Auvergne and the Eifel, and notably in the Island of Ascension, where the mineral forms beautiful tabular crystals.
The eastern ranges of the Bolivian Andes are formed of Palaeozoic rocks with granitic and other intrusions; the Western Cordillera consists chiefly of Jurassic and Cretaceous beds, together with the lavas and ashes of the great volcanoes; while the intervening plateau is covered by freshwater and terrestrial deposits through which rise ridges of Palaeozoic rock and of a series of red sandstones and gypsiferous marls of somewhat uncertain age (probably, in part at least, Cretaceous).