353 
M A R G A; 
left frequently obferved in it, fucb js grypliitto., bekmnites, 
pe&inites, chamites, ammonites, &c» and fometimes alfo 
vegetable impreflions. 
A remarkable and beautiful variety of indurated marl, 
which by fome writers is -confidered as a kind'of compaft 
limeftone, is that known under the name of Florence marble , 
or ruin marble , of which the following defcription is given 
by Brard. It prefents angular figures of a. yellowilh- 
brown, cn a bale of a lighter tint, and which pafles, in 
•diminifhing, to a whitifli grey. Seen at a certain diftance, 
flabs of this ftone refemble drawings done in biltre. One 
is amufed to obferve in it kinds of ruins ; there it is a 
Gothic caitle half deftroyed, here i't prefents ruined walls, 
in another place old baftions; and what ftill adds to the 
iliufion is, that in thefe forts of natural paintings there 
exifts a kind of aerial perfpetlive, which is very fenfibly 
perceived. The lower part, or what forms the firft plane, 
has a warm and bold tone; the fecond follows it, and 
weakens as it increafes its diftance 5 the third becomes dill 
fainter ; while the upper part, agreeing with the firlt, pre¬ 
fents in the diftance a whitifh zone, which terminates the 
horizon, then blends itfelf more and more as it rifes, and 
at length reaches the top, where it fometimes forms, as it 
were, clouds. But approach clofe to it, all vanifhes im¬ 
mediately j and thofe pretended figures, which, at a diftance, 
feemed fo well drawn, are converted into irregular fpots, 
which prefent nothing to the eye. This play of nature is 
owing to ferruginous infiltrations in the fifiures of this 
marble, which otherwife is of dull fraCture and very ar¬ 
gillaceous ; whence it is never ufed in architecture; they 
merely make flabs of it, which are framed like little pic¬ 
tures, and which are much efteemed in commerce when of 
certain dimenfions. It fometimes occurs that the fame 
flab is fawed in two, and the parts are fet together in the 
fame frame, fo as to appear but as one piece ; and the 
drawings on the right and left bear a refemblance which 
flill further helps the iliufion. There are fome who, to 
outdo Nature, put painted figures at the bottom of thefe 
pictures ; but this is an exuberance of the wonderful, 
which finiflies by fpoiling all. 
Mr. Brard fhould have alfo mentioned a variety of 
the fame Florentine marble, which, inltead of the ruins, 
exhibits fine'black dendritae, arranged in fuch a manner 
as to reprefent, in the molt deceptive manner, groups of 
trees, fhrubs, See. This is called landscape marble. 
To this indurated marl muft alfo be referred the fine va¬ 
riety called Cottam marble, from being found at Cottam, 
near Briftol. It refembles, in many reipefts, the lalt-men- 
tioned variety of Florentine marble; its ground is afh- 
coloured or bluifh-grey, with blackifti-brown fpots and 
veins, moft of which have a dentritic form, reprefenting 
affemblages of Ihrubs and trees, grottoes, &c. It is found 
in large oblong pieces. 
The fingular balls, known by the name of Lucius Hel- 
montii, belong likewife to indurated marl. They are of 
various fizes, from one inch to nearly one foot and a half 
in diameter, and generally difpofed in a regular manner in 
beds of marl. When broken in a diredtion parallel to one 
of- the largeft furfaces, their interior exhibits a number of 
Allures by which the whole mafs is divided into perfeftly- 
diftindt, and more or lefs regular, prilins of from three to 
fix or more fides; the Allures themfelves being fometimes 
empty, but oftener filled up with another fubftance, which 
is generally granular limeftone in a cryftalline (late. There 
can be no doubt that the fingular conformation of the in¬ 
terior of thefe balls is the refult of a kind of contraction 
produced by exficcation; but it is difficult to account for 
the regularity of the fifiures, their not extending to the 
furface of the balls, and their being Itill filled with fub- 
ftanees which it would be moft natural to fuppofe in¬ 
jected from without. Thefe balls are found in almoft all 
countries where marl is not unfrequent. Thofe of Fran¬ 
conia and of Antwerpen are very regular in their internal 
flruCture; but the Ifle of Sheppey, in-the county of Kent, 
VOL. XIV. No. 979. 
produces the largeft and fineft of them. They are alfo 
found of con fid enable fize, and of a bluilh-grey colour, in 
Derbylliire. Thofe of Durham are divided into fmall 
liexahedral columns of a very regular form and equal fize, 
while the narrow nftures feparating the prifms from each 
other are filled with quartz. When the marl is difengaged 
from the quartzy fepta, thefe latter more or lefs reprefent 
the figure of a honey-comh, 
6. Marga bituminofa, or bituminous marl: indurated, 
not crumbling in the air; black ; a little greafy, firming 
a little within, of a fiaty texture, breaking into difeoid 
fragments. Found in ftratified mountains of various parts 
of Germany, Italy, and Swilferland ; frequently contain¬ 
ing the impreftion of filh and marine plants, and frequently 
the ores or oxydes of copper; colour greyilh, bluilh, or 
brownifti-black, according to the quantity of bitumen it 
contains, which renders it more or lefs inflammable; has 
a greafy and fomewhat-glittering appearance, and a fiaty 
texture ; the thin plates are a little fonoroivs; burns be¬ 
fore the blow-pipe with a black drofs. The varieties rich 
in copper are often regularly wrought as ores of this me¬ 
tal ; no other ufe is made of bituminous marl fiate, which, 
when decompofing in the air, forms an earth injurious to 
vegetation. 
Bituminous marl is fubordinate to the fletz-limeftone 
formation, in which beds are fometimes formed by it. 
Its lowermolt ftratum, which refts on the old fandllone, 
is generally rich in copper ores, whence it has received 
the name of copper-Jlate ; thefe ores are copper pyrites, vi¬ 
treous copper ore, variegated copper ore, more leldom 
copper azure, copper-green, malachite, and flill more rarely 
native copper. Befides thefe, alfo fome galena and car¬ 
bonated iron is found in it; and its rifts are fometimes 
coated with felenile. In this fiate are frequently found 
the impreflions and remains of fifties converted into coal, 
and which, by the convulfed and contorted attitudes ira 
which they are feen, appear to indicate a fudden cataf- 
trophe, by which whole flioals of them periftied. Em- 
merlin indeed goes fo far as to fuppofe that the copper 
pyrites, often found in marl of this kind, was the occafion 
of their death, their fubftance itfelf being impregnated 
with the pyritical matter. Sauflure mentions a curious 
fadt refpdtling the impreffions of fifii met with on a hard 
fchiftofe marl in Monte Bolca, near Verona ; namely, that 
the impreflions of one hundred and live different lpecks 
have been there recognized, of which 
Twenty-feven belong to European feas. 
Thirty-nine 
Three 
Eighteen 
Eleven 
Seven - 
Afiatic feas. 
African leas. 
South American feas. 
North American feas. 
Frelh-water lakes and 
rivers of different parts of the world. 
The quantity of fifties in the marl-flate of Thuringia is 
generally proportionate to the quantity of copper it con¬ 
tains ; fometimes the whole of thefe remains is converted 
into copper pyrites. In the bituminous marl-llate of Ri- 
chelfdorf, in Heffia, Mr. Ries has difeovered fome remark, 
able impreffions which he confiders as produced by the 
bones of a child’s hand ; but, according to profeffor Blu- 
menbach, the bones belonged to animals of another order 
of mammalia. The bituminous marl-llate pafles into in¬ 
durated marl; fometimes alfo into ftinkftone. 
7. Marga anonyma, or limeftone-marl: {billing within, 
hardilh, of a dull iron-colour. Found near St. Amboife 
in France ; lometimes fo hard as fo admit a polilh and 
ftrike fire with fteel; befides a little fchiftofe earth, ful- 
phur, and quartz, it contains iron ^, argil carbonat 
of lime 
MAR'GA (St.), in geography, a.fmall ifland of Hun¬ 
gary, in tlie Danube: ten miles fouth of Buda. 
MAR'GA SEER'SHA, an Hindoo'month which partly 
agrees with our October. 
M 4 R'GAB> 
1 
