Rhyolite

Changes of

Ground-mass.—The ground-mass of rhyolites is liable to other changes, of which tile most important are silicifica tion, kaolinization and sericitization. Among the older rocks of this group it is the exception to find that secondary quartz has not been deposited in some parts of them. Often indeed the matrix is completely replaced by silica in the form of finely crystalline quartz or chalcedony; and these rocks on analysis prove to con tain over 9o% of silica. In the recent rhyolites of Hungary, New Zealand, etc., the deposit of coarse opal in portions of the rock is a very common phenomenon.

Kaolinization may be due to weathering, and the stony dull look of the matrix of many microcrystalline rhyolites is due to the decomposed state of the felspar grains in them; it is even more typically developed by fumarole action, which replaces the felspars with soft, cloudy white products belonging to a mineral of the kaolin group. Sericitization, or the development of fine white mica after felspar, is usually associated with shearing, and is com monest in the older rhyolites.

Vesicular structure is very common in rhyolites; in fact the pumiceous obsidians have this character in greater perfection than any other rocks (see PumicE); but even the felso-rhyolites are very often vesicular. The cavities are usually lined with opal and tridymite ; in the older rocks they may be filled with agate and chalcedony. The "mill-stone porphyries," extensively used in Germany for grinding corn, are porous rhyolites ; the abundance of quartz makes them hard, and their rough surfaces render them peculiarly suitable for this purpose. In some of them the cavities are partly secondary. These rocks' are obtained in the Odenwald, Thuringerwald and Fichtelgebirge.

Occurrence.

In Britain a pale grey Tertiary rhyolite occurs at Tardree, Antrim, and in Skye. Felsitic rhyolites occur among the Old Red rocks of Scotland (Pentland Hills, Lorne, etc.), in Devonshire, and in large numbers in North Wales. The Carnar vonshire rhyolites are of ten much altered and silicified; many of them have a nodular structure which is very conspicuous on weathered surfaces. The spheroids may be two or three in. in diameter; some are built up of concentric shells. Rhyolites are also known from Fishguard, Malvern, Westmorland and Co. Waterford. One of the oldest volcanic rocks of Britain (pre Cambrian, Uriconian) is the spherulitic rhyolite of the Lea Rock near Wellington, Shropshire. It shows bright red spherulites in great numbers and is probably an obsidian completely denitrified. Perlitic structure is also visible in it.

In

other parts of Europe rhyolites have a fairly wide distribu tion though they are not numerous. In Hungary (Hlinik, etc.) there are many well-known examples ; they extend along the margin of the Carpathians and are found also in Siebenburgen. In Italy they occur in the Euganean Hills and in the Lipari Islands; the latter being the principal source of pumice at the present day. Rhyolites of Recent age occur in Iceland (Myvatn, etc.), where they are characterized by the frequent absence of quartz, and the presence of much plagioclase and pyroxene. Some of these rocks have been called trachyte-obsidians, but they seem to be rhyolites which contain an exceptionally large amount of soda. The older rhyolites, which are generally called quartz porphyries in Germany, are mostly of Permian or Carboniferous age and are numerous in the Vosges, Odenwald, Thuringerwald, etc. They are often accompanied by basic rocks (melaphyres).

Permian rhyolites occur also at Lugano in Italy. Rhyolites are known also in Asia Minor and the Caucasus, in New Zealand, Col orado, Nevada and other parts of western North America. In the Yellowstone National Park there is a well-known cliff of obsidian which shows remarkably perfect columnar jointing. Some of the rhyolites of Nevada are exceedingly rich in porphyritic minerals, so that they appear at first sight to be holocrystalline rocks, since the ground-mass is scanty and inconspicuous. To this type the name nevadite has been given, but it is rare and local in its distribution.

In Pantellaria, south-west of Sicily, there are rocks of rhyolitic affinities which present so many unusual features that they have been designated pantellarites. They contain less silica and alumina and more alkalis and iron than do ordinary rhyolites. Their fel spars are of the anorthoclase group, being rich in soda together with potash, and are very variable in crystalline development. Aegirine-augite and forms of soda-amphibole are also character istic of them, while dark brown aenigmatite or cossyrite often occur. Quartz is not very plentiful; other ingredients are olivine, arfvedsonite and tridymite. The ground-mass varies much, being sometimes quite vitreous, at other times a glass filled with swarms of microliths, while in certain pantellarites it is a microcrystalline aggregate of quartz and alkali felspar. The absence of plagioclase and biotite are marked distinctions between these rocks and the rhyolites, together with the scarcity of quartz and the prevalence of soda-bearing pyroxenes and amphiboles. ComQndites are prac tically identical rocks. They occur in Sardinia, Corsica, British East Africa, East Siberia, West Texas and the East Indies.

Among the Palaeozoic volcanic rocks of Germany there is a group of lavas, the quartz-keratophyres, which are of acid com position and rich in alkali felspar. Their dominant alkali is soda : hence their felspars are albite and 'cryptoperthite, not sanidine as in rhyolites. Quartz occurs sometimes as corroded phenocrysts, but is often scarce even in the ground-mass. Porphyritic biotite or augite are very rare, but occur in the matrix along with felspars and quartz. Micropegmatite is not infrequent in these rocks, and they may be silicified like the rhyolites. As quartz-keratophyres mostly occur in districts where there has been a good deal of f old ing, they are often crushed and more or less sericitized. They are best known from the Devonian rocks of Westphalia and the Harz, but are also found in New South Wales, and similar rocks have been described (as soda-felsites) from Ireland. The rocks which they accompany are usually dolerites and spilites.

Composition.

The following analyses show the composition of some of the principal types of rhyolites We note in the rhyolites the very high silica, with alkalis and alumina also in considerable amount, while lime, magnesia and iron are very low. In the pantellarite, keratophyre and comendite the silica tends to be less abundant, while the alkalis, especially soda, increase ; they have less alumina but are richer in iron and magnesia. It is easy to see why the latter types contain less quartz, felspars often very rich in soda, and ferric minerals which contain iron and alkalis in notable amounts such as aegirine, riebeckite and arfvedsonite. ( J. S. F.)