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See also: petrology, a See also: group of volcanic rocks which consist mainly of sanidine (or glassy See also: orthoclase) See also: felspar
.
Very often they have minute irregular steam cavities which make the broken surfaces of specimens of these rocks rough and irregular, and from this character they have derived their name
.
It was first given by }fatly to certain rocks of this class from See also: Auvergne, and was long used in a much wider sense than that defined above, in fact it included See also: quartz-trachytes (now known as liparites and rhyolites) and See also: oligoclase-trachytes, which are now more properly assigned to andesites
.
The trachytes are often described as being the volcanic equivalents of the plutonic syenites
.
Their dominant See also: mineral, sanidine felspar, very commonly occurs in two generations, i.e. both as large well-shaped porphyritic crystals and in smaller imperfect rods or laths forming a finely crystalline groundmass
.
With this there is practically always a smaller amount of See also: plagioclase, usually oligoclase; but the potash felspar (sanidine) often contains a considerable See also: pro-portion of the soda felspar, and has rather the characteristics of anorthoclase or cryptoperthite than of pure sanidine
.
Quartz is typically absent from the trachytes, but See also: tridymite (which likewise consists of See also: silica) is by no means uncommon in them
.
It is rarely in crystals large enough to be visible without the aid of the microscope, but in thin slides it may appear as small hexagonal plates, which overlap and See also: form dense aggregates, like a mosaic or like the tiles on a roof
.
They often cover the surfaces of the larger felspars or See also: line the steam cavities of the See also: rock, where they may be mingled with amorphous See also: opal or fibrous chalcedony
.
In the older trachytes secondary quartz is not rare, and probably sometimes results from the recrystallization of tridymite
.
Of the ferromagnesian minerals See also: present See also: augite is the most See also: common
.
It is usually of pale See also: green colour, and its small crystals are often very perfect in form
.
See also: Brown
See also: hornblende and See also: biotite occur also, and are usually surrounded by black corrosion See also: borders composed of See also: magnetite and See also: pyroxene
.
Some-times the replacement is See also: complete and no hornblende or biotite is See also: left, though the outlines of the cluster of magnetite and augite may clearly indicate from which of these minerals it was derived
.
See also: Olivine is unusual, though found in some trachytes, like those of the Arso in Ischia
.
Basic varieties of plagioclase, such as See also: labradorite, are known also as phenocrysts
in some See also: Italian trachytes
.
Dark brown varieties of augite and rhombic pyroxene (See also: hypersthene or See also: bronzite) have been observed but are not common
.
See also: Apatite, See also: zircon and magnetite are practically always present as unimportant See also: accessory minerals
.
The trachytes being very See also: rich in potash felspar, necessarily contain considerable amounts of alkalis; in this character they approach the phonolites
.
Occasionally minerals of the felspathoid group, such as See also: nepheline, See also: sodalite and See also: leucite, occur, and rocks of this kind are known as phonolitic trachytes
.
The soda-bearing amphiboles and pyroxenes so characteristic of the phonolites may also be found in some trachytes; thus aegirine or aegironic augite forms outgrowths on See also: diopside crystals, and riebeckite may be present in spongy growths among the felspars of the groundmass (as in the See also: trachyte of Berkum on the Rhine)
.
Trachytic rocks are typically porphyritic, and some of the best-known examples, such as the trachyte of Drachenfels on the Rhine, show this character excellently, having large sanidine crystals of See also: tabular form an inch or two in length scattered through their See also: fine-grained groundmass
.
In many trachytes, however, the phenocrysts are few and small, and the ground-mass comparatively coarse
.
The ferromagnesian minerals rarely occur in large crystals, and are usually not conspicuous in See also: hand specimens of these rocks
.
Two types of ground-mass are generally recognized: the trachytic, composed mainly of long, narrow, sub-parallel rods of sanidine, and the orthophyric, consisting of small, squarish or rectangular prisms of the same mineral . Sometimes granular augite or spongy riebeckite occurs in the groundmass, but as a See also: rule this See also: part of the rock is highly felspathic
.
Glassy forms of trachyte (obsidians) occur, as in See also: Iceland, and pumiceous varieties are known (in See also: Teneriffe and elsewhere), but these rocks as contrasted with the rhyolites have a remark-ably strong tendency to crystallize, and are rarely to any considerable extent vitreous
.
Trachytes are well represented among the See also: Tertiary and See also: Recent volcanic rocks of See also: Europe
.
In Britain they occur in See also: Skye as See also: lava flows and as dikes or intrusions, but they are much more common on the continent of Europe, as in the Rhine See also: district and the See also: Eifel, also in Auvergne, Bohemia and the Euganean Hills
.
In the neighbourhood of See also: Rome, Naples and the See also: island of Ischia trachytic lavas and tuffs are of common occurrence
.
In See also: America trachytes are less frequent, being known in S
.
Dakota (Black Hills)
.
In Iceland, the See also: Azores, Teneriffe and See also: Ascension there are Recent trachytic lavas, and rocks of this kind occur also in New See also: South See also: Wales (Cambewarra range), See also: East See also: Africa, See also: Madagascar, See also: Aden and in many other districts
.
Among the older volcanic rocks trachytes also are not scarce, though they have often been described under the names orthophyre and orthoclase-porphyry, while "trachyte " was reserved for Tertiary and Recent rocks of similar composition
.
In See also: England there are See also: Permian trachytes in the Exeter district, and Carboniferous trachytes are found in many parts of the central valley of Scotland
.
The latter differ in no essential respect from their See also: modern representatives in See also: Italy and the Rhine valley, but their augite and biotite are often replaced by See also: chlorite and other secondary products
.
Permian trachytes occur also in Thuringia and the See also: Saar district in See also: Germany
.
Closely allied to the trachytes are the Keratophyres, which occur mainly in Palaeozoic strata in the Harz (Germany), in the See also: Southern Uplands of Scotland, in See also: Cornwall, &c
.
They are usually porphyritic and fluidal; and consist mainly of See also: alkali felspar (anorthoclase principally, but also See also: albite and orthoclase), with a small quantity of chlorite and iron oxides
.
Many of them are lavas, butfor a lengthy monograph on a subject, dealing with it technically and authoritatively, whereas a See also: tract is understood to be brief and rather argumentative than educational
.
There is, again, the rarer word tractate, which is not a tract, in the precise sense, so much as a See also: short See also: treatise
.
The word " tract " has come to be used for brief discourses of a moral and religious character only, and in modern practice it seems to be mainly confined to serious and hortatory themes
.
An essay on See also: poetry, or the description of a passage of scenery, would not be styled a tract
.
In the See also: Protestant See also: world, the tract which See also: Luther composed in 1520, on the Babylonish captivity, has been taken more or less as the type of this See also: species of literature, which, however, existed long before his See also: day, both in Latin and in the vernacular tongues of western Europe
.
It is difficult, if not impossible, in early See also: history, to distinguish the tract from other cognate forms of moralizing literature, but it may perhaps be said that the homilies of A3lfric (955-1025?) are the earliest specimens of this class in See also: English literature
.
Four centuries later Wyclif issued a series of tracts, which were remarkable for their vigour, and exercised a strong influence on See also: medieval See also: theology
.
See also: Bishop Reginald Pecock published many controversial tracts between 1440 and 1460
.
See also: Sir See also: Thomas More,
See also: John
See also: Fisher (d
.
1535) and See also: William Tyndale were prominent writers of controversial
See also: treatises
.
It was the See also: Martin Marprelate agitation, in the reign of
See also: Elizabeth, which led from 1588 to 1591 to the most copious production of tracts in English literature; of these nearly
See also: thirty survive
.
On the Puritan See also: side the See also: principal writers were John Udall (1560-1592), See also: Henry
See also: Barrowe (d
.
1593), John See also: Penry (1559-1593) and See also: Job Throckmorton (1545-1601), the tracts being printed in the See also: house of the last-mentioned; on the side of the Established See also: Church the principal authors were Bishop Thomas
See also: Cooper (1517-1594) and the poets Lyly and
See also: Nash
.
An enormous collection of tracts was published between 1717 and 1720 in elucidation of what is known as the Bangorian Controversy, set in motion by a See also: sermon of Benjamin See also: Hoadly, bishop of See also: Bangor, on " The Nature of the See also: Kingdom of Christ " (1717)
.
Convocation considered this a treatise likely to impugn and impeach the royal supremacy in religious questions
.
A vast number of writers took part in the dispute, and Thomas Sherlock (1678-1761) See also: fell into disgrace through the violence of his contributions to it
.
Convocation was finally obliged to give way
.
The most famous collection of tracts published in the course of the 19th century was that produced from 1833 onwards by Newman, See also: Keble and E
.
B
.
See also: Pusey, under the title of " Tracts for the Times." Among these Pusey's " Tract on See also: Baptism" (1835) and his " On the See also: Holy Eucharist" (1836) had a profound effect in leading directly to the foundation of the High Church party, so much so that the epithet " Tractarian " was barbarously coined to designate those who wished to oppose the spread of rationalism by a quickening of the Church of England
.
In 1841 Newman's " Tract No
.
XC." was condemned by the heads of houses in See also: Oxford, and led to the definite organization of the High Church forces
.
(X.)
Tract See also: Societies are agencies for the production and distribution, or the distribution only, of Christian literature, more especially in
SiO2 Al20, Fe202 FeO MgO CaO Na2O See also: K20 See also: H2O
Riebeckite trachyte, Hohenberg, Berkum, Rhenish Prussia
.
. 66•o6 16.46 2.25 1.10 0.19 0.79 6.81 5.52 0.62
Keratophyre, See also: Hamilton
See also: Hill,
See also: Peebles, Scotland 64.38 16.98 4.04 - 0'28 1.08 7.57 4.30 1.64
Trachyte (Orthophyre) Garleton Hill, See also: Haddington,Scotland
.
. 61.35 16.88 0.41 5.01 0.44 2.39 5.26 6.12 1.70
Trachyte, See also: Monte Nuovo, Phlegraean See also: Fields, near Naples, Italy
.
.
.
60.33 18.74 2.84 1.29 0.38 1.15 7.15 7.30 0.56
Trachyte, Algersdorf, Bohemia 64.69 18.39 - 3.44 0.49 1.72 4.61 6.46 0.24
others are probably dikes or thin intrusions
.
As the analyses given above will show, they differ from trachytes mainly in being richer in soda
.
U
.
S
.
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