HOCKS. 
cm 
Mies. The si/e of the grains varies mach: 
v, hen large, the rock is usually called pudd- 
ing slope. 
’I his rock is very distinctly stratified. The 
beds of it often alternate with beds of com- 
pact limestone, pit coal, oolite, and a species 
of sandstone shistus fine grained, and mixed 
with l.-aves of mica, which gives it the ap- 
pearance of micaceous shistus. It contains 
tew metallic ores of value. Sometimes co- 
balt is found in it. 
Secondary limestone. This rock is simple, 
and composed of compact limestone. Oc- 
casional iy it contains crystals of quartz, py- 
rites, See. Shells occur very frequently in 
it. It is very distinctly stratified. Its beds 
are sometimes separated by beds of shistose 
bituminous marl and sandstone, and by tu- 
bercles of hornstone and flint often arranged 
in beds. It is often traversed by metallic 
veins, chiefly of galena, grey copper ore, 
malachite, &c. 
Chalk , The strata of chalk may be con- 
sidered perhaps as subordinate to those of 
secondary limestone. They .consist entirely 
cf chalk, sometimes interrupted b;> thin beds 
of flint in tubercles. Shells often occur con- 
verted into siliceous matter, and sometimes 
pyrites in spherical masses. No metallic ores 
ever occur in them. 
Gypsum. The strata of gypsum usually 
occur in, mountains alternating with those of 
sandstone, limestone, marl, clay, rock salt. 
Tliey often contain foreign crystals ; chiefly 
quartz, arragonjte, boraciie, garnet : some- 
times they contain sulphur. Few petrefac- 
tions are found in them except the bones of 
quadrupeds. They contain scarcely any me- 
tallic ores. See SULPHA! OF LIME. 
Rock salt. The mountains which contain 
strata of rock salt are to be referred to a 
particular formation of gypsum, with which 
they usually alternate. They contain no 
ores. 
Pit coal is found k two different forma- 
tions. The first of .these is distinguished 
particularly by the name of coal formation, 
or mountains of coal. They are usually 
composed of beds of, 1. Very brittle sand- 
stone, containing often small particles of 
mica; 2 Another sandstone or pudding stone 
ot very large grains: 3. Shistose day; 4. 
Marl 5. Limestone ; 6. An argillaceous 
porphyry distinguished by the name of se- 
condary^ porphyry ; 7. Ferruginous clay ; 8. 
Coal. "The bed's of coal vary in thickness 
and in number. 
This formation of coal occupies countries 
of no great elevation. They occur chiefly 
at the bottom of chains, and in the inter- 
mediate valleys. The strata of coal in the 
north of England belong to it. 
Coal is found in other situations, especially 
in the secondary trap formation. Those of 
Scotland belong to this class. 
Argillaceous ironstone. 1 he beds of this 
mineral usually alternate with those of indu- 
rated clay, shistose clay, marl, brandschiefer, 
and sandstone. They* frequently contain ca- 
lamine mixed with galena.. The impressions 
of plants and marine petrefactions are often 
observable in them. They usually form 
small insulated hills, and are not very com- 
mon. 
Secondary traps. The mountains of se- 
condary traps are composed of various rocks ; 
some of wince belong exclusively to this for- 
mation ; others are found also in other moun- 
tains. The rocks peculiar to secondary traps 
are, 1. Basalt; 2. VYacken ; 3. Basaltic tufa ; 
4. Secondary inandiestein ; 5. Porphyry shis- 
tus ; 6. Graustein; and, 7. Secondary grun- 
stein. 
1. Basalt, considered as the mass of a 
mountain, is a rock more or less compound ; 
usually it is of a porphyritic structure, has lor 
its ground the mineral called basalt. It con- 
tains usually grains of olivine, augite, basaltic 
hornblende, magnetic ironstone, and some- 
times lcucite, felspar, quartz, &c. ; sometimes 
also mica, aetinote, chalcedony. Sometimes 
it assumes tne structure of mandelstein. In 
that case its cavities are idled with zeolite, 
steatites, limestone, &c. 
it usually appears in large separate masses 
often prismatic. It is very common often 
forming detached mountains. 
2. Waeken sometimes forms beds in the 
secondary traps. It is usually between clay 
and basalt. It neither contains olivine nor 
augite, but crystals of basaltic hornblende, 
and above all of biack hexahedral mica. This 
last substance distinguishes waeken from ba- 
salt, which very seldom contains it. 
3. Basaltic tufa results from the decompo- 
sition of certain basalts. It consists of frag- 
ments of basalt, pieces of olivine, the remains 
of vegetables, ike. agglutinated by a cement 
of clay. 
4. The amygdaloids or mandelsteins of the 
secondary traps have tor their base a clav 
which seems to be a decomposed grunstein, 
often penetrated with siliceous matter. It 
has a good deal of resemblance to waeken, 
and sometimes passes into it : at other times 
it assumes a more compact texture, and pass- 
es into basalt. Its cavities are sometimes 
empty, sometimes tilled with green earth, zeo- 
lite, limestone, &c. 
5. Porphyry shistus is a rock whose structure 
is shistose, and its texture porphyritic. Its 
base is klingstein, containing grains of felspar 
and sometimes of hornblende. It has a good 
deal of resemblance to basalt, and often passes 
into it But it is more nearly a chemical 
compound, being more transparent, sonorous, 
and Lard. 
6. Graustein is a rock composed of small 
grains of felspar and hornblende, which gra- 
duate into each other and form a mass al- 
most homogeneous of an ash grey colour. It 
contains olivine and augitg. 
7. Secondary grunstein, like the primitive 
and transition, is composed of hornblende 
and felspar ; but its grains are less perfectly 
crystallised, and less intimately mixed, it 
usually covers rocks of basalt. 
Such are the rocks peculiar to the second- 
ary traps. They have all less or more of a 
crystallized structure ; whereas the other mi- 
nerals found in secondary traps, but not pe- 
culiar to them, are mechanical depositions. 
Waeken and basalt form the passage from the 
one to the other. The mechanical deposi- 
tions, sand, clay, & c. are usually lowermost ; 
they are covered by waeken. Some of the 
other substances always occupy the summits. 
Trap mountains contain abundance of pe- 
trefactions, but no ores except some veins 
and grains of iron. They are usually insu- 
lated; very seldom forming chains. The 
mountains of basalt and porphyry shistus are 
usually conicak They are hardly -ever cover- 
ed by other rocks. They usually cover sand- 
stone, coal, secondary limestone, shi s loti 
clay ; and veins of them are not uncommon 
in primitive mountains. 
CLASS V. 
Rocks volcanic. 
This name is given to all the minerals 
thrown out during volcanic eruptions. The 
most complete account of them has been 
given by Dolomieu, who devoted the greatest 
part ot bis life to the study of volcanoes. 
Bart of his division was published by him in 
the Journal de Physique for 1794 ; and an 
abstract of the whole has been given by Bro- 
chant from notes taken during a course of 
geology given by Dolomieu in 1797. Vol- 
canic products have been divided bv this 
celebrated geologist into live classes : i . Mi- 
nerals modified by the fire ot volcanoes ; 2. 
Substances not modified by the fire, or thrown 
out unaltered ; 3. Substances sublimed by 
the fire of volcanoes ; 4. Minerals altered-by 
the sulphurous acid vapours of volcanoes ; 
5. Volcanic minerals altered by the action of 
the atmosphere. 
Minerals modified by the fire. This set 
of minerals is subdivided into two heads. The 
first comprehends those minerals which, 
though they have been modified by 'fire' vet 
exhibit no appearance of its action. The se- 
cond consists of those which retain obvious 
marks of the action of lire. Those of the 
first head have been distinguished bv the 
name of compact lavas ; those of the second 
by the name of porous. 
Compact lavas bear so close a resem- 
blance to certain rocks of an origin not vol- 
canic, that it is extremely difficult to distin- 
guish them. Some have the appearance of 
basalt; others of granite, porphyry, and va- 
rious secondary traps. Dofoinieu divides 
them into four species according to their 
base. 
1. Compact lavas with an arglllo-ferrugi- 
nous base Their colour is usually black. 
Fracture imperfectly conchoidal. Texture 
very compact. Sonorous. Smell argillace- 
ous. Attract the magnetic needle. Very 
common in volcanic countries. Frequently 
contain crystals of augite, felspar, hornblende, 
garnet, leucite, olivine, mica. A specimen 
of the lava of Catania in Sicily, analysed by 
Dr. Kennedy, contained, 
51.0 silk a, 
19.0 alumina, 
1 4. 5 oxide of iron, 
9.5 lime, 
4.0 soda, 
1.0 muriatic acid, 
99.0. 
A specimen of the lava of Sta. Venere in 
Sicily he found to contain 
55.78 silica, 
17.50 alumina, 
14.25 oxide of iron 
10.00 lime, 
4.00 soda, 
1.00 muriatic acid, 
97.5. 
Compact lava with a petrosiliceous base. 
Colour very variable, grey, black, and white ; 
but all become white before the blow-pipe. 
Fracture conchoidal. Grain fine and com- 
pact. Smell slightly argillaceous. Does not 
affect the magnetic needle. Contains usually 
