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group cordierite also occurs, not rarely, and may have the outlines of imperfect hexagonal prisms which are divided up into six sectors when seen in polarized See also:

LIGHT

light. In See also:

BIOTITE

biotite hornfelses a faint striping may indicate the See also:

ORIGINAL

original bedding of the unaltered See also:

rock and corresponds to small changes in the nature of the sediment deposited. More commonly there is a distinct spotting, visible on the surfaces of the See also:

hand specimens.. The spots are See also:

ROUND (O. Fr. rond, Lat. rotundus, the Fr. is the source also of Du. rond; Ger., Swed., Dan. and Nor. rond)

round or elliptical, and may be paler or darker t han the See also:

REST (O. Eng. rest, reste, bed, cognate with other Teutonic forms, e.g. Ger. Rast, Riiste, rest, and probably Gothic Rasta, league, i.e. resting or stopping place)

rest of the rock. In some cases they are See also:

rich in See also:

GRAPHITE

graphite or carbonaceous matters; in others they are full of See also:

brown See also:

MICA

mica; some spots consist of rather coarser grains of quartz than occur in the matrix. The frequency with which this feature reappears in the less altered slates and hornfelses is rather remarkable, especially as it seems certain that the spots are not always of the same nature or origin. " See also:

TOURMALINE

Tourmaline hornfelses " are found sometimes near the margins of tourmaline granites; they are See also:

black with small needles of See also:

SCHORL

schorl which under the See also:

MICROSCOPE (Gr. µucp6s, small, asenreiv, to %view)

microscope are dark brown and richly pleochroic. As the tourmaline contains See also:

BORON (symbol B, atomic weight I I)

boron there must have been some permeation of vapours from the See also:

GRANITE (adapted from the Ital. granite, grained; Lat. granum, grain)

granite into the''sediments.

Rocks of this group are often seen in the Cornish See also:

TIN (Lat.- stannum, whence the chemical symbol " Sn "; atomic weight =117.6, 0=16)

tin-See also:

MINING

mining districts, especially near the lodes. A second See also:

GREAT

great group of hornfelses are the See also:

TALC

talc-silicate-hornirises which arise from the thermal alteration of impure See also:

lime-stones. The purer beds recrystallize as See also:

MARBLES

marbles, but where there has been originally an admixture of See also:

sand or See also:

clay lime-bearing silicates are formed, such as See also:

DIOPSIDE

diopside, See also:

EPIDOTE

epidote, See also:

garnet, See also:

SPHENE

sphene, See also:

VESUVIANITE

vesuvianite, See also:

SCAPOLITE (Gr. O'Kairos, rod, )tLOoc, stone)

scapolite; with these See also:

PHLOGOPITE

phlogopite, various felspars, See also:

PYRITES

pyrites, quartz and actinolite often occur. These rocks are See also:

FINE

fine-grained, and though often banded are tough and much harder than the original limestones. They are excessively variable in their mineralogical See also:

composition, and very often alternate in thin seams with biotite See also:

HORNFELS (a German word meaning hornstone)

hornfels and indurated quartzites. When perfused with boric and fluoric vapours from the granite they may contain much See also:

AXINITE

axinite, fluorite and See also:

DATOLITE

datolite, but the aluminous silicates (andalusite, &c.) are absent from these rocks. From diabases, basalts, andesites and other igneous rocks a third type of hornfels is produced. They consist essentially of See also:

FELSPAR, or FELDSPAR

felspar with See also:

HORNBLENDE

hornblende (generally of brown See also:

COLOUR (Lat. color, connected with celare, to hide, the root meaning, therefore, being that of a covering)

colour) and See also:

PALE (through Fr. pal, from Lat. palms, a stake, for paglus, from the stem pag- of pangere, to fix; " pole " is from the same original source)

pale See also:

PYROXENE

pyroxene. Sphene, biotite and See also:

IRON [symbol Fe, atomic weight 55.85 (0=16)]

iron oxides are the other See also:

COMMON

common constituents, but these rocks show much variety of composition and structure. Where the original See also:

mass was decomposed and contained See also:

CALCITE

calcite, See also:

ZEOLITES

zeolites, See also:

CHLORITE

chlorite and other secondary minerals either in See also:

VEINS

veins or in cavities, there are usually rounded a reas or irregular streaks containing a See also:

SUITE (Suite de pieces; Ordre; Partita)

suite of new minerals, which may resemble those of the talc silicate hornfelses above described. The original porphyritic, fluidal, vesicular or fragmental structures of the igneous rock are clearly visible in the less advanced stages of hornfelsing, but become less, evident as the alteration progresses. In some districts hornfelsed rocks occur which have acquired a schistose structure through shearing, and these See also:

FORM (Lat. forma)

form transitions to See also:

SCHISTS (Gr. vxq'ecv, to split)

schists and gneisses which contain the same minerals as the hornfelses, but have a schistose instead of a hornfels structure.

Among these may be mentioned cordierite and sillimanite gneisses, andalusite and kyanite mica schists, and those schistose talc silicate rocks which are known as cipolins. That these are sediments which have undergone thermal alteration is generally admitted, but the exact conditions under which they were formed is not always clear. The essential features of hornfelsing are ascribed to the See also:

ACTION

action of See also:

HEAT (0. E. haktu, which like " hot," Old Eng. hat, is from the Teutonic type haita, hit, to be hot; cf. Ger. hitze, heiss; Dutch, hitte, heet, &c.)

heat, pressure and permeating vapours, regenerating a rock mass without the See also:

PRODUCTION (Lat. productionem, from producere, to pro-duce)

production of See also:

FUSION

fusion (at least on a large See also:

scale). It has been argued, however, that often there is extensive chemical See also:

CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change owing to the introduction of See also:

MATTER

matter from the granite into the rocks surrounding it. The formation of new felspar in the hornfelses is pointed out as pursued amid innumerable difficulties his purpose of self-See also:

EDUCATION

education. His university career lasted three years, and on its termination he became a See also:

TUTOR (Lat. tutor, guardian, tueri, to watch over, protect)

tutor at Toxteth, devoting to astronomical observations his brief intervals of leisure. In 1636 he met with a congenial spirit in See also:

William Crabtree, a See also:

draper of See also:

Broughton, near See also:

Manchester; and encouraged by his See also:

ADVICE (Fr. avis, from Lat. ad, to, and visum, viewed)

advice he exchanged the guidance of Philipp von Lansberg, a pretentious but in-accurate Belgian astronomer, for that of See also:

KEPLER, JOHANN (1571-1630)

Kepler. He now set himself to the revision of the Rudolphine Tables (published by Kepler in 1627), and in the progress of his task became convinced that a transit of See also:

VENUS

Venus overlooked by Kepler would nevertheless occur on the 24th of See also:

NOVEMBER (Lat. novem, nine)

November (O.S.) 1639. He was at this See also:

time See also:

CURATE (from the Lat. curare, to take care of)

curate of See also:

HOOLE, JOHN (2727-1803)

Hoole, near See also:

Preston, having recently taken orders in the See also:

Church of See also:

England, although, according to the received accounts, he had not attained the canonical See also:

AGE (Fr. age, through late Lat. aetaticum, from aetas)

age. The 24th of November falling on a See also:

SUNDAY

Sunday, his clerical duties threatened fatally to clash with his astronomical observations; he was, however, released just in time to See also:

witness the punctual verification of his forecast, and carefully noted the progress of the phenomenon during See also:

HALF

half an See also:

HOUR

hour before sunset (3.15 to 3'45). This transit of Venus is remarkable as the first ever observed, that of 1631 predicted by Kepler having been invisible in western See also:

EUROPE

Europe. Notwithstanding the See also:

RUDE, FRANCOIS (1784–1855)

rude character of the apparatus at his disposal, Horrocks was enabled by his observation of it to introduce some important corrections into the elements of the See also:

PLANET (Gr. ssXavirrns, a wanderer)

planet's See also:

ORBIT (from Lat. orbita, a track, orbis, a wheel)

orbit, and to reduce to its exact value the received estimate of its apparent See also:

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

diameter.

After a See also:

YEAR

year spent at Hoole, he returned to Toxteth, and there, on the See also:

eve of a See also:

long-promised visit to his friend Crabtree, he died, on the 3rd of See also:

JANUARY

January 1641, when only in his twenty-second year. To the inventive activity of the discoverer he had already See also:

UNITED

united the patient skill of the observer and the See also:

PRACTICAL

practical sagacity of the experimentalist. Before he was twenty he had afforded a specimen of his See also:

POWERS, HIRAM (1805-1873)

powers by an important contribution to the lunar theory. He first brought the revolutions of our See also:

SATELLITE (from the Lat. satelles, an attendant)

satellite within the domain of Kepler's See also:

LAWS

laws, pointing out that her apparent irregularities could be completely accounted for by supposing her to move in an See also:

ELLIPSE (adapted from Gr. EXkei 'tc, a deficiency, iXXeirely, to fall behind)

ellipse with a variable eccentricity and directly rotatory See also:

major See also:

AXIS (Lat. for " axle ")

axis, of which the See also:

earth occupied one See also:

FOCUS (Latin for " hearth " or " fireplace ")

focus. These precise conditions were afterwards demonstrated by See also:

Newton to follow necessarily from the See also:

law of See also:

GRAVITATION (from Lat. gravis, heavy)

gravitation. In his speculations as to the See also:

PHYSICAL

physical cause of the See also:

CELESTIAL, OR STELLAR

celestial motions, his mind, though not wholly emancipated from the tyranny of gratuitous assumptions, was working steadily towards the light. He clearly perceived the significant See also:

ANALOGY (Gr. avaXo-yLa, proportion)

analogy between terrestrial gravity and the force exerted in the See also:

SOLAR, SOLLER (Lat. solarium, Fr.• galetas, Ital. solaio)

solar See also:

SYSTEM

system, and by the ingenious See also:

DEVICE

device of a circular pendulum illustrated the composite character of the planetary movements. He also reduced the solar See also:

PARALLAX (Gr. aapa?X6, alternately)

parallax to 14" (less than a See also:

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

quarter of Kepler's estimate), corrected the See also:

sun's semi-diameter to 15' 45", recommended decimal notation, and was the first to make tidal observations. Only a remnant of the papers See also:

LEFT

left by Horrocks was preserved by the care of William Crabtree. After his See also:

DEATH

death (which occurred soon after that of his friend) these were See also:

PURCHASED

purchased by Dr Worthington, of Cambridge; and from his hands the See also:

TREATISE

treatise Venus in See also:

SOLE (Solea)

sole visa passed into those of See also:

HEVELIUS [HEVEL or H6WELCKE], JOHANN (1611—1687)

Hevelius, and was published by him in 1662 with his own observations on a transit of See also:

Mercury. The remaining fragments were, under the directions of the Royal Society, reduced by Dr See also:

WALLIS, JOHN (1616-1703)

Wallis to a compact form, with the heading Astronomia Kepleriana defensa et promota, and published with numerous extracts from the letters of Horrocks to Crabtree, and a See also:

SKETCH (directly adapted from Dutch schets, which was taken from Ital. schizzo, a rough draft, Lat. schedium, something hastily made, Gr. oxE&os, sudden, off-hand, axeb6v, near by; Ger. Skizze and Fr. esquisse are from the same source)

sketch of the author's See also:

LIFE

life, in a See also:

VOLUME

volume entitled Jeremiae Horroccii See also:

OPERA (Italian for " work ")

opera posthuma (See also:

LONDON

London, 1672). A memoir of his life by the Rev.

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