1911 Encyclopædia Britannica/Variolites

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VARIOLITES (Lat. variola, smallpox), in petrology, a group of dark green basic igneous rocks which, especially on weathered surfaces, exhibit pale coloured spots that give them a pockmarked appearance. In some conditions these spots weather out prominently; they are grey, pale green, violet or yellowish, while the matrix of the rock is usually dark green. The variolites are related most closely to the basalts or diabases. They are nearly always much decomposed, and, since they are also ine-grained rocks, their original composition may be much obscured by secondary changes. The variolitic spots are rounded in outline and are often about a quarter of an inch in diameter, but may much exceed this size. They have a radiate structure and are sometimes, though not generally, zoned with concentric circles, of different appearance and composition. Many authors have compared them with the spherulites of the acid rocks (obsidian's and rhyolites), and undoubtedly some kinds of variolite are merely glassy spherulitic varieties of basalt. The tachylyte selvages of the dolerite dikes of the west of Scotland, for example, often contain large brown spherulites which are easily visible in hand specimens. These spherulites consist of very thin divergent fibres, and their nature is often difficult to determine on account of the indefiniteness of the optical characters of minerals in this state. It seems probable, however, that they are mostly felspar embedded in dark brown glass. Small phenocrysts or skeleton crystals of olivine, augite and plagioclase felspar may occur in these tachylytes.

Other variolites are glassy or partly crystalline facies of olivine-free dolerites, occurring as thin dikes or intrusions, or at the margins of dolerite masses. In these the felspars are well crystallized as thin rods, with square or forked ends, radiating outwards from a centre. They are commonly oligoclase, and sometimes assume branching or feathery forms. Some authors would call these “ sphaero-crystals " rather than spherulites; they are an intermediate stage between the latter and the stellate groupings of felspar which occur frequently in igneous rocks. In the same rocks augite spherulites occur also, but this mineral forms plumose growths, branching and curved, which spread through the glassy base and do not interfere with the felspar spherulites. They have much resemblance to the feathery ice crystals which form on window-panes. Occasionally olivine-dolerites have a coarsely spherulitic structure with long rods of plagioclase felspar converging to a point; one example of these rocks from Skye contains variolites over 'three inches in diameter.

Another group of variolites includes the most famous rock of this type, which comes from the Durance, in France. Pebbles of this were well known to collectors for a long time before they were traced to their source at Mont Genevre. They were proved to belong to a diabasic rock which shows well-marked “ pillow-structure ” or “ spheroidal jointing." Each pillow has a marginal portion which is variolitic, but towards the centre of the block-shaped masses the structure becomes coarse and groups of radiate felspars make their appearance. It is doubtful whether the variolite is an intrusive rock or a lava flow. Many of these pillow lavas (or spilites) occur in the Devonian rocks of Germany, and often they have variolitic facies which seem to belong to the same group as the rock of the Durance. Their spherulites are very often oligoclase felspar or decomposition products after a felspathic mineral. In other cases they consist of chlorite or pale green amphibole, both of which may be secondary after pyroxene. The ground mass is very fine grained and is filled with chlorite, epidote, leucoxene, and other secondary minerals. There is much reason to believe that it was originally in large measure vitreous but has suffered devitrifaction. Sometimes little steam cavities occur and may serve as a nucleus from which the variolite has grown. The radiate structure of the varioles is often nearly obliterated in these much-decomposed rocks, in fact it may never have been very perfect. Variolites are found also in several parts of the Swiss Alps at latluga on Lake Onega, in Anglesey, the Lleyn district and Fishguard in Wales, in Cornwall, and in more than one place in Ireland.

Finally, there is a group of spotted rocks formerly known to French petrographers as the variolites du Drac from the locality in which they are found, but they have been proved to be merely vesicular, rotten diabases, with steam cavities filled with white calcite and other secondary minerals.  (J. S. F.)