Others are palagonite-breccias with the vacuoles of the altered glass filled with opal.
The prevailing rocks are tuff-breccias and agglomerates.
In their character they pass on the one hand into the foraminiferous volcanic mud-rocks or clay rocks and on the other into the tuff-breccias and tuff-agglomerates.
Inclosed in these breccias are many angular and hardened fragments of laminated clay in different states of alteration by heat, and intermixed with volcanic sands.
Here numerous proofs are seen of submarine eruptions, by which the argillaceous and sandy strata were invaded and cut through, and tufaceous breccias formed.
Among the fragments which abound in the tufaceous breccias of Somma, none are more common than a saccharoid dolomite, supposed to have been derived from an ordinary limestone altered by heat and volcanic vapours.
Talcose granite, or Protogine of the French, is a mixture of felspar, quartz, and talc.
It also passes in the same uninterrupted manner into a basalt, and at length into a soft claystone, with a schistose tendency on exposure, in no respect differing from those of the trap islands of the western coast.
The former of these is an aggregate of schorl, or tourmaline, and quartz.
When felspar and mica are also present, it may be called schorly granite.
Syenite, however, after maintaining the granitic character throughout extensive regions, is not uncommonly found to lose its quartz, and to pass insensibly into syenitic greenstone, a rock of the trap family.
A] It would be easy to multiply examples and authorities to prove the gradation of the granitic into the trap rocks.
The opening of new fissures in the hardest rocks is a frequent accompaniment of such convulsions, and during the consequent vibrations, breccias must often be caused.
Lower Permian sandstones and breccias of Penrith and Dumfriesshire, intercalated.
The hollows in some of the fragments of vesicular lava of which the breccias and conglomerates are composed are partially filled with calc-sinter, being thus half converted into amygdaloids.
Professor Ramsay refers the angular form and large size of the fragments composing thesebreccias to the action of floating ice in the sea.
Penrith sandstone, as it has been called, and the associated breccias and purple shales are estimated by Professor Harkness to attain a thickness of 3000 feet.
Such breccias may have been partly the result of the subaerial waste of an old land-surface which gradually sank down and suffered littoral denudation in proportion as it became submerged.
He remarks that similar breccias with slickensides are observed on a minor scale where rocks of different composition and rigidity are contorted together.
It is well exhibited in cliffs and reefs along the shore, by breccias and conglomerates, thin shales, yellow and greenish sandstones and flags, and concretionary limestone.
It is possible that some of the breccias or conglomerates may be referred to aqueous causes, as great floods occasionally sweep down the flanks of Etna, when eruptions take place in winter, and when the snows are melted by lava.
Their account is peculiarly interesting to geologists, because it throws light on the red osseous breccias containing the bones of extinct quadrupeds which are so common in almost all the countries bordering the Mediterranean.
In this manner we may account for those bony breccias which we often find in caves, some of which are of high antiquity while others are very recent and in daily progress.
The water of these torrents is charged with pebbles and red ochreous earth, resembling precisely the well-known cement of the osseous breccias of the Mediterranean.
The alternating breccias are made up of scoriae, sand, and angular blocks of lava.
The base of this series is often constituted by massive breccias with included fragments of the older rocks, upon which they may happen to repose; and similar breccias sometimes occur in the upper portion of the series as well.
These cavern-deposits are of very various nature, consisting of mud, loam, gravel, or breccias of different kinds.
Of the breccias there is a great variety among the relics of ancient Rome.
It is to these breccias we owe the volcanic appearances in the Truckee River Canyon, a few miles before reaching the Lake.
Towards the summits the breccias gradually lose their stratified character and become more firmly cemented.
There are several layers of the andesites breccias at the head of Bear Creek Canyon, above Deer Park Springs.
Over large areas in the Truckee quadrangle the andesite masses consist of breccias containing numerous dykes and necks of massive andesite.
Similar breccias occur in the south of Scotland, and these are stated to be "overlain by a deposit of glacial age, so similar to the breccia below as to be with difficulty distinguished from it.
Permian breccias are also found in Ireland, containing {90} blocks of Silurian and Old Red sandstone rocks which Professor Hull believes could only have been carried by floating ice.
But between the breccias of Haddingtonshire and the equivalent deposits in Berwickshire there is no space for any intermediate range of mountains of circumdenudation of such a height.
The limestone-breccias of Gibraltar have been described by Professor Ramsay and myself, and we have shown that these could only have been formed under the influence of excessive frost and melting snows.
Breccias which have probably had a similar origin occur also in Corsica, Malta, and Cyprus, and doubtless they will yet be recognised in many other places.
DM] It seems to me, then, that these breccias are in every way better accounted for by a lowering of temperature due to increased eccentricity of the orbit.
Take, for example, the case of the breccias and conglomerates of the Lammermuir Hills, which have all the appearance of being glacial and fluvio-glacial detritus.
The breccias of the Permian system have been described by Ramsay as of glacial origin.
It was composed, where I could examine it, of the calcareous breccias of the sea-coast.
The slow retreat of the waters has turned into dry ground this extensive plain, in which rises a group of small hills, composed of gypsum and calcareous breccias of very recent formation.
Rocks in the foreground and to the right are rudely bedded volcanic breccias of the Clarno Formation.
The pinnacles known as "Rabbit Ears," above the prominent talus in figure 15, are of vent breccias that consist mostly of welded blocks of scoriaceous basalt, but also contain volcanic bombs which were blown out as blobs of fluid lava.
Volcanic ashes and breccias are accompanied by devitrified pitchstones and intruded granitic rocks, which may or may not be all of the same general age[55].
Footnote 96: It should be mentioned that some writers have inferred the evidence of glacial conditions over parts of the British area, on account of the resemblance of some of the Permian breccias to recent glacial deposits.
The rocks of the St Davids ridge consist of a binary granite (granitoidite), felsites, and volcanic ashes and breccias of intermediate composition.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "breccias" in a variety of sentences. We hope that you will now be able to make sentences using this word.
