Earths. M 1 N E R A L O G Y. Earths. 
456 
to the tongue ; receives an imperfeCt polifli; hardens in 
the air and in oil, and when faturated with the latter makes 
an excellent whetftone. 
2. Ardeiia tabularis, or writing-flate : a little polifhed, 
foft, greyilh-black, making a whitifh mark, with ftraight 
foliations. Found in Swillerland, Hungary, Franconia, 
and Saxony; admits an imperfect polifli, and is l'ometimes 
variegated with darker orbicular or oblong fpots ; when 
powdered, effiervefces in a very flight degree with nitric 
acid; is rather light, and is ufed for tables and writing- 
Hates. 
3. Ardefia tegularis, or common (late : a little polilhed, 
rather hard, bluifh-black with a cinereous ftreak, and 
ftraight foliations. It varies in colour, as reddifli, purple, 
and greenifli-grey. Found in many mountains of Great 
Britain, and various parts of F.urope, generally in layers, 
and frequently marked with the impreflion of living-bodies 
and plants ; when broken, fliines a little from a mixture 
of micaceous particles or granulations of quartz ; does 
not adhere ro the tongue, or imbibe water; and is prin¬ 
cipally ufed for the covering of houfes. There is a very 
exteniive quarry of flate at Stonesfield, near Woodftock, 
which has the property of being eafily feparated into la¬ 
minae by mechanical means, or even by the aCtion of the 
atmofphere. The property of being thus eafily feparated 
into laminae depends partly upon the proportion of clay 
contained in it, (for this property is in general more 
remarkable in proportion to the quantity of clay con¬ 
tained,) and partly upon the nature of its original de- 
polition ; for the ftone of fome parts of the quarry con¬ 
tains a conliderable quantity of minute fliells refembling 
millet-feeds; and it feems worth obfervation, as connected 
with the fchiftofe property of the ftone, that the depofi- 
tion of fliells is more abundant on the furface than in the 
fubftance of the laminae. 
4. Ardeiia folida, or hard flate : blackifli, of a compaCt 
flaty texture, giving a clear found when ftruck, making a 
cinereous ftreak. Found in Sweden, Spain, and New 
Spain; of a fliivery frafiture, and a blackilh, brown, grey, 
or reddifli, colour, 
5. Ardefia compnCtiffima, or very-compafit flate : of a 
dull colour, very compact and folid, hardilh, leaving a 
whitilh ftreak. Found very rarely in China; black or 
brown, exteriorly glabrous and unequal, internally very 
compact. 
6. Ardefia atrata, or black flate : of a lamellar flaty 
texture, very foft, making a whitifh ftreak. Found in 
Lapland, and various provinces of Sweden; meagre, 
blackilh, brown, or yellowifh, crackling when ftirred in 
the fire, and running into a frothy kind of glafs in a greater 
degree of heat, effervefcing a little with nitric acid when 
powdered. 
7. Ardefia undulata, or undulated flate : black, of an 
undulated flaty texture. Found in Finland and Jemtia : 
l'ometimes fofter, and melting into porous flags; fome- 
times a little harder, and melting into a folid glafs. 
8. Ardefia bituminofa, or bituminous flate ; very foft, 
alittle greafy, of a dull colour, filming when rubbed, leav¬ 
ing a black ftreak, of a ftraight flaty texture, breaking into 
difcoid fragments, linoking or flaming, and becoming 
paler, in the fire. Found in Britain, Sweden, and Lufatia ; 
forming large beds in ftratified mountains ; and is ftrong- 
ly impregnated with bitumen and fulphur pyrites. Co¬ 
lour generally black, a little glittering when broken ; 
when expofed to heat, it fmokes or flames, emits a bitumi¬ 
nous odour, and becomes paler after loling its bitumen. 
9. Ardefia kellas, or killas : of a fibrous texture, flight- 
ly adhering to the tongue. Found in Cornwall; of a pale 
bluifh grey, red, or whitifh-yellow, colour, and often in¬ 
terfered with veins of copper or tin. Surface undulated; 
fraCture long, fplintery, imperfefitly flaty; luftre opake, 
fllky. Contains filex 60, argil 25, magnefia 9, iron 6, 
and fome petroleum or bitumen. 
10. Ardefia nigrica, or black chalk : deep black, mea¬ 
gre, very foft, foiling the fingers, making a black ftreak ; 
of an incurved flaty texture, breaking into difcoid -frag* 
ments or long fplinters, becomingreddifh-grey in the fire. 
Found in Weftrogothia, Franconia, and Italy, in folid 
mafles, without luftre; adheres flightly to the tongue, feels 
fmooth, aflumes a polifli from a knife, gives a black ftreak 
and marks black, does not readily moulder in water, or 
effervefce or diffolve in acids, when heated to rednefs be¬ 
comes reddifh-grey. Contains filex 64-60, alumine 11-25, 
charcoal iroo, oxyd of iron 2-75, water 7-50. This l’ub- 
ftanc'e is a confiderable object of traffic in Italy; and 
hence by artifts is fometimes called pierre d'Italic, or Ita¬ 
lian ftone. 
11. Ardefia teflularis, or rhombic fchiftus : very foft, 
fomewliat ponderous, breaking into trapezoid fragments, 
of a flaty texture. Found in Silelia near Goldberg and 
Neudorf, forming entire mountains; of a brown, pale 
yellow,- or green, colour. 
Basaltes.—C onfifting of a large proportion of filex, 
with a fmaller proportion of alumine and oxyd of iron, 
and often a little lime, magnefia, oxyd of manganefe, and 
foda ; opake, inconfpicuous, meagre, generally becom¬ 
ing greyifii when rubbed with a knife, breaking into in¬ 
determinate fragments, mouldering in the air into argil; 
not effervefcing with nitric acid, melting before the blow¬ 
pipe into a black glafs attracted by the magnet. 
The origin of the name bafalt.es is dubious ; and its ap¬ 
plication is by no means well-defined; for Dolomieu ob- 
ferves, that many fubltances of a different nature fo much 
refemble each other in analyfis as well as appearance, that 
it is difficult to fix the application of the name to any one. 
The fame obfervation holds with refpeCt to the luhin-Jione 
of Scotland. It would be convenient therefore, fays Dr. 
Kidd, to conlider balalt and whin-ftone as fynonymous ;• 
and to give a feparate defcription of various fubftances 
that have been conlidered as fubordinate to thefe ; or, 
wdiere the fubftances themfelves cannot be afcertained, 311 
explanation of their names. 
That all bafalt has been originally in a ftate of igneous 
fufion is highly probable; and it appears alfo likely, 
from the experiments of Mr. Gregory Watt, that the ba¬ 
falt in cooling cryftallized in a confufed manner round 
numerous central points within its fubftance ; that the 
cry Utilization radiating from thefe central 'points in all 
directions produced numerous lpherical mafles; the pro- 
cels of cryftaljization flopping as foon as the radii of two 
adjoining cryftalline centres had extended fufliciently to 
come into contafit with each other. But, as he obferved 
alfo that thefe radii did not proceed independently of each 
other, but advanced periodically by regularfleps to points 
equidiftant from the centre ; thus forming a radiated and 
at the fame time a concentric ftruCture ; he conceives 
that the difintegration of bafalt of this kind, proceeding 
in an inverted order, from the circumference towards the 
centre, would give rife to the production of thofe fpherical 
mafles ; and this opinion is confirmed by the evident ap¬ 
pearance of a concentric difintegration that occafionally 
takes place. Stirling-caftle Hands on a bafaltic rock re¬ 
markably difpoled to undergo the difintegration above- 
defcribed. 
The low degree of heat at which bafalt melts was once 
made the ground of an argument againft that part of Dr. 
Hutton’s theory which luppoles that this fubftance was 
conlblidated from a ftate of igneous fufion ; for it was ar¬ 
gued that in this cafe bafalt would in its characters have 
refembled the black glafs into which by heat it may eafily 
be converted. But the force of this argument has been 
completely fubverted by the refults of the experiments of 
fir James Hall. By thefe it has been proved that, of the 
fame mafs of bafalt reduced to a uniform glafs, any part 
will retain the vitreous character if cooled rapidly; and 
any part, if cooled very flowiy, will refume a texture and 
appearance fo nearly refembling the original rock from 
which it was detached, that even an experienced eye fhall 
fcarcely be able to deteCl a difference. Certainly there 
will be no greater difference than might be found between 
two 
