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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 See also:

FINE

fine-grained rocks, their See also:

ORIGINAL

original See also:

composition may be much obscured by secondary changes. The variolitic spots are rounded in outline and are often about a See also:

QUARTER (through Fr. from Lat. quartarius, fourth part)

quarter of an See also:

INCH (O. Eng. ynce from Lat. uncia, a twelfthpart; cf. " ounce," and see As)

inch in See also:

DIAMETER (from the Cr. &a, through, µErpov, measure)

diameter, but may much exceed this See also:

SIZE

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 See also:

SPHERULITES (Gr. cchaipa, sphere, Mhos, stone)

spherulites of the See also:

ACID (from the Lat. root ac-, sharp; acere, to be sour)

acid rocks (obsidians and rhyolites), and undoubtedly some kinds of variolite are merely glassy spherulitic varieties of See also:

BASALT

basalt. The tachylyte selvages of the See also:

DOLERITE (from Gr. SoXepos, deceptive)

dolerite dikes of the See also:

west of See also:

Scotland, for example, often contain large See also:

brown spherulites which are easily visible in See also:

hand specimens. These spherulites consist of very thin divergent See also:

fibres, and their nature is often difficult to determine on See also:

account of the indefiniteness of theoptical characters of minerals in this See also:

state. It seems probable, however, that they are mostly See also:

FELSPAR, or FELDSPAR

felspar embedded in dark brown See also:

glass. Small phenocrysts or See also:

SKELETON

skeleton crystals of See also:

OLIVINE

olivine, See also:

AUGITE

augite and See also:

PLAGIOCLASE

plagioclase felspar may occur in these See also:

TACHYLYTES, or TACHYLITES (from Gr. Taxbs, swift, Xbew, to dissolve, meaning " easily fused," though some have erroneously interpreted it as " easily soluble in acids ")

tachylytes. Other variolites are glassy or partly crystalline facies of olivine-See also:

FREE

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 See also:

OLIGOCLASE

oligoclase, and sometimes assume branching or feathery forms. Some authors would See also:

CALL (from Anglo-Saxon ceallian, a common Teutonic word, cf. Dutch kallen, to talk or chatter)

call these " sphaero-crystals " rather than spherulites; they are an intermediate See also:

STAGE (Fr. 6/age; from Lat. stare, to stand)

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 See also:

MINERAL

mineral forms plumose growths, branching and curved, which spread through the glassy See also:

BASE

base and do not interfere with the felspar spherulites. They have much resemblance to the feathery See also:

ICE (a word common to Teutonic languages; cf. Ger. Eis)

ice crystals which See also:

FORM (Lat. forma)

form on window-panes. Occasionally olivinedolerites have a coarsely spherulitic structure with See also:

long rods of plagioclase felspar converging to a point; one example of these rocks from See also:

SKYE

Skye contains variolites over three inches in diameter. Another group of variolites includes the most famous rock of this type, which comes from the See also:

DURANCE (anc. Druentia)

Durance, in See also:

France. Pebbles of this were well known to collectors for a long See also:

time before they were traced to their source at Mont Genevre. They were proved to belong to a diabasic rock which shows well-marked " See also:

PILLOW (O. Eng. pylu; Lat. pulvinus, a cushion)

pillow-structure " or " spheroidal jointing." Each pillow has a marginal portion which is variolitic, but towards the centre of the See also:

block-shaped masses the structure becomes coarse and See also:

groups of radiate felspars make their appearance. It is doubtful whether the variolite is an intrusive rock or a See also:

LAVA

lava flow. Many of these pillow lavas (or spilites) occur in the Devonian rocks of See also:

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 See also:

CHLORITE

chlorite or pale green See also:

AMPHIBOLE

amphibole, both of which may be secondary after See also:

PYROXENE

pyroxene. The ground See also:

mass is very fine grained and is filled with chlorite, See also:

EPIDOTE

epidote, leucoxene, and other secondary minerals. There is much See also:

REASON (Lat. ratio, through French raison)

reason to believe that it was originally in large measure vitreous but has suffered devitrifaction. Sometimes little See also:

steam cavities occur and may serve as a See also:

NUCLEUS (Lat. for the kernal of a nut, nux, the stone of fruit)

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 See also:

ALPS

Alps at Jatluga on See also:

Lake See also:

ONEGA

Onega, in See also:

Anglesey, the Lleyn See also:

DISTRICT

district and See also:

FISHGUARD (Abergwaun)

Fishguard in See also:

WALES (Cymru, Gwalia, Cambria)

Wales, in See also:

CORNWALL

Cornwall, and in more than one See also:

PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place in See also:

Ireland. Finally, there is a group of spotted rocks formerly known to See also:

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 See also:

white See also:

CALCITE

calcite and other secondary minerals. (J. S.

End of Article: VARIOLITES (Lat. variola, smallpox)

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