ROC 
ROL 
grains. of felspar; sometimes of hornblende, 
mica : Lucites are uncommon in it. 
Compact lavas with a granite base. So 
named because they resemble granite, and 
contain all its constituents. The felspar usu- 
ally predominates. It is in lamellar masses, 
seldom in crystals. This species contains 
crystals" of hornblende, mica, augite. 
Compact lavas with a leucite base. These 
lavas are uncommon. No current of lava 
has ever been observed composed entirely of 
them. They occur near Vesuvius. In them 
the leucites are so abundant and compressed, 
that they assume a compact appearance. 
They sometimes contain crystals of- horn- 
blende, augite, mica. 
Compact lavas like basalt often affect a 
prismatic form. 
Porous lavas, fyc. This second head con- 
sists according to Dolomieu, of minerals hav- 
ing the same base with compact lavas ; but 
these bases have undergone certain modifi- 
cations, in which the action of fire has be- 
come evident. 'I hese modifications are 
chiefly three ; swelling, vitrification, and cal- 
cination. The minerals belonging to this 
head may be reduced to seven species, 
1. Porous lavas. The lavas on the surface 
of currents usually assume this form ; espe- 
cially the surface of the argillo-ferruginous 
lavas. 'The cavities are usually spheric in 
them, while in the porous lavas, formed from 
the other compact lavas, the cavities are 
commonly elongated. They are often em- 
ployed as millstones and in buildings. 
2. Seorias. d he substances distinguished 
by this name have more or less resemblance 
to metallic seorias. To them may be referr- 
ed the substance called black pouzzolano. It 
has been produced by scorification, though 
it no longer retains the characters of it. 
The term pouzzolano (derived from the 
city Pouzzoles), has been usually applied to 
earthy matters ejected by volcanoes, which 
make an excellent mortar with lime. See 
Puzzolana. The best is found always in 
the antient currents ; that in the modern 
forms bad mortar. There are three sorts of 
pouzzolano ; the black, which is a scoria al- 
tered : the white, which is composed of pu- 
mice; and the red, which belongs to the pro- 
ducts of calcination. 
Compact glasses. Volcanic vitrifications 
are uncommon, especially compact glasses. 
They all resemble common glass. They 
are more or less transparent, often black, 
sometimes blueish or greenish, very seldom 
colourless. Sometimes prismatic. 
Porous glasses or pumice stones. When 
the compact glasses are exposed to the heat 
of our furnaces, they emit a great number of 
air-bubbles, which renders them porous. 
Such is the prig in of pumice. It lias the same 
base as compact glass. The texture of pu- 
mice stones is fibrous ; the fibres have a silky 
lustre. Colours various ; white, brown, yel- 
low, black. But before the blow-pipe they 
all melt into a white enamel. White pouzzo- 
lano is composed of the detritus of these 
stones. A specimen of pumice stone analys- 
ed by Klaproth, yielded 
77.a0 silica, 
1 7.50 alumina, 
1 .75 Gxide of iron, 
3.00 soda of potass. 
99.75. 
Roe 
Nearly the same result had been previous- 
ly obtained by Dr. Kennedy. 
Volcanic sands and ashes. The sands are 
composed of grains varying i« size. They 
are usually mixed with crystals of felspar, 
augite, magnetic ironstone, &c. and often 
cover a great extent of ground. Etna has 
covered the country for 50 leagues round it 
with a bed of sand twelve feet thick. Vol- 
canic ashes are merely very fine sand. They 
are so light, that during the eruptions of 
Etna, the wind often transports them as far as 
Egypt. 
Agglutinated matters. These are merely 
sands and ashes covered and cemented toge- 
ther by a torrent of melted lava. 
Calcined substances. All stony bodies 
which have undergone a kind of calcination 
by volcanic fires are denoted by this name. 
All volcanic matters often undergo this 
change. Their grain is rendered more harsh, 
and tiieir feel more dry. The ferruginous 
lavas become more red, and cease to be at- 
tracted by the magnet. 
Minerals not modified by the fire . These 
matters existed in the mountain before the 
commencement of volcanic fire, and are 
thrown out by it unaltered. The study of 
them is important, because they inform us of 
the internal structure of volcanic mountains. 
They usually belong to the primitive rocks. 
Sometimes they are fragments of rocks, and 
sometimes groups of crystals. They are 
thrown out in general at the beginning of 
eruptions. 
Volcanoes sometimes emit torrents of 
muddy water. From these have originated 
the minerals called volcanic tufas. Their 
colour is various. 
Substances sublimed. An immense quan- 
tity of matter is exhaled by volcanoes ; partly 
in the state of gas or steam, partly in a visible 
form. 
Hydrogen gas, carbonic acid, sulphurous 
acid, muriatic acid, nitric acid, &c. have been 
detected issuing from them. 
The mineral substances which are sublimed 
from them, and which afterwards are depo- 
sited on their sides, are sulphur, which is 
very abundant; mineral oil, and various salts, 
especially muriats of ammonia, soda, copper, 
and iron ; sulphats of alumina, soda, iron, 
and copper, and carbonat of soda. Metallic 
bodies are also found among these substan- 
ces, iron,copper, antimony, arsenic, cinnabar, 
&TC. 
‘Substances altered by sulphurous acid va- 
pours. The sulphur volatilized by volca- 
noes is often converted into an acid", which, 
acting upon the lavas, changes their appear- 
ance considerably. They become of a yel- 
lowish-white colour, much lighter and dryer, 
and are more easily pulverized. They con- 
tain an unusual proportion of silica, because 
the sulphurous acid has combined with the 
alumina, and formed a salt afterwards washed 
away by the rain. The tame vapours often 
attack stony matters not volcanic. 
The principal products of the action of 
these vapours on lavas are alum and sul- 
phats of lime, magnesia and iron. These 
salts, especially the' first, are collected with 
great advantage. 
Volcanic substances altered by the action 
oj the atmosphere. All rocks undergo great- 
er or smaller changes when long exposed to 
the atmosphere ; but these changes are much 
4G * 
dot 
greater and more rapid in voleauie rocks 
than in others. Sometimes, however, it. is 
very slow. Hence the age of volcanoes can- 
not be determined by tiie state of volcanic 
eruptions. 
The argillo-ferruginous lavas become first 
red ; the petrosiliceous become of a dirty 
grey. By degrees they assume an earthy ap- 
pearance, and pass at last to a kind of friable 
clay. 1 lie seorias undergo the same changes 
much more rapidly. The earthy matters pro- 
duced by this decomposition are afterwards 
washed down by the waters, and form 
large beds, which constitute a very fertile 
soil, I he porous lavas are often partly fill- 
ed with earth washed down by rains from de- 
composed lavas. Dolomieu supposed many 
crystals to owe their existence to the infiltra- 
tions of such waters. 
Besides the real products of volcanoes, 
there are rocks which have been more or lessr 
altered by the action of fires not volcanic, 
t hese lires have often originated from the" 
combustion of strata of coal. These have 
been called pseudo-volcanic rocks. These 
locks are four in number; namely, porcelain 
jasper, burnt clay, earthy seorias, and a par- 
ticular variety ofpolierschiefer. 
Porcelain jasper is considered as a shistose* 
clay, which has been calcined. Burnt clav 
resembles brick: It has been exposed to a 
weaker fire than porcelain jasper. Like that 
mineral it is considered as having been origi- 
nally a shistose clay. Earthy seorias an* 
light porous substances like seorias. They 
appear to have been melted. They are usu- 
ally near burnt coal strata. A variety of 
polierschiefer sometimes occurs, which" ap- 
pears to have been a clay exposed to a mode- 
rate degree oi heat, and rather dried thaw 
calcined. I o these pseudo-volcanic miner- 
als may be added the vitrified sorts, not im- 
common in the highlands of Scotland. They 
seem to have originated from artificial fires. 
See Geology, Mountains, Mineralo- 
gy, & c. 
ROD a land measure of sixteen feet and 
a halt : the same with perch and pole. 
tW' ln .8 au S in g* See Gauging. 
KUE, the spawn or seed of fish. That of 
male fishes is usually distinguished bv the 
name of soft-roe, or milt, and that of the fe- 
male, by hard-roe, or spawn. 
So inconceivably numerous are these ovula 
or small eggs, that M. Petit found 342,144 of 
them in a carp of eighteen inches ; but Mr 
Leewenhoeck found in a carp no more than 
21 1,629. This last gentleman observes, that 
there are four times this number in a cod, and 
oue c:on tains 9,344,000 eggs. 
ROELLA, a genus of the monogvnia 
order, m the peutandria class of plants ; "and 
in the natural method ranking under the 
twenty-ninth order, cam panacea;. The co- 
rolla is funnel-shaped, with its bottom shut up 
by stanuinferous valvules; the stigma is bi- 
fid; the capsule bilocular, and "cylindrical 
inferior. There are five species/ shrubby 
plants of the Cape. J 
ROGL E, in law. See Vagrant. 
R1IORIA, a genus of the class and order 
tnandria moiiogynia : the cal. is bell-shaped 
five-petal led, unequal ; stigmas three, revo^ 
lute ; caps. There is one species, a shrub 
of Guiana. 
ROLANDRA, a genus of the class and 
order syngemjsia polygamia superflya. Ti*p 
Vol. II. 
