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Although most of the transition rocks appear to have been 
formed from a state of solution, and therefore possess the crys¬ 
talline character, yet their crystallization appears to have 
been less perfect than in the primitive rocks, because the 
parts of which they are composed have a lower degree of 
lustre, inferior hardness, less translucency, and colours of less 
purity, than primitive rocks. In short, on a general view, 
we should say, these rocks have more of the earthy aspect, 
and of the fragmented character, thus intimating a lower de¬ 
gree of crystallization, than is observable in the primitive 
class. But transition rocks are further distinguished from 
those of the primitive class, by the very important circum¬ 
stance of their containing fossil organic remains. These pe¬ 
trifactions are of corals and shells, animal productions low in 
the zoological scale; and of vegetable remains that appear to 
belong to plants of the most simple construction, such as 
those of the class cryptogamia,.and therefore at the bottom of 
the botanical scale. Hence it follows, that animals and ve¬ 
getables of the more simple construction, were, those first 
called into existence, and that their creation did not take 
place until the period of the formation of transition rocks! 
Corals of the same nature as those met with in the preceding 
class also occur in transition rocks; and numerous and exten¬ 
sive beds of limestone, sometimes containing organic remains, 
are in some districtsof frequent occurrence. Transition rocks 
frequently abound in ores of various descriptions, which are 
generally disposed in veins. The mining districts of the 
Leadhills and Wanlock-head, near Edinburgh, which are so 
rich in galena, or lead-glance, are situated in transition rocks. 
The rich lead and silver mines in the Hartz, and many of 
those in Mexico, are in rocks of the same description. 
The gems which appear in so many interesting forms and 
relations in the strata and veins of primitive mountains, are 
comparatively rare in the present class. 
The abundant occurrence of ores in this class of rocks; the 
extensive deposites of limestone, particularly of the variegated 
kinds, so highly prized for ornamental purposes, which they 
contain; the fine granites and porphyries which they afford, 
are sufficient proofs of their importance in the arts. 
The following are the rocks belonging to this class, viz. 
greywacke, clay-slate, limestone, trap, granite, sienite, por¬ 
phyry, serpentine, gneiss, mica-slate, and quartz rock. 
1. Greywacke is a conglomerated looking rock, with a 
basis of clay-slate, including angular and various shaped 
portions (by many considered as fragments) of clay-slate, of 
fiinty-slate, quartz, felspar, &c. and occasionally scales of 
mica. When the imbedded masses become small, and the 
mass slaty, it is named greywacke-slate. 
2. Clay-slate. —This rock is of the same general nature 
with primitive clay-slate, but differs from it in having less 
lustre, and in sometimes containing fossil plants and fossil 
shells. 
3. Limestone. —It is more compact, and has much smaller 
grains, and therefore has less lustre and lower translucency 
than the primitive limestone. It is frequently traversed by 
veins of calcareous spar, and often exhibits in the same bed 
various tints and shades of beautiful colours. Some varieties 
are conglomerated, forming the brecciated marble of artists, 
and others contain fossil shells and corals. 
4. Trap. —'This rock, like that of the primitive class, is 
principally composed of hornblende, and is sometimes asso¬ 
ciated with felspar, forming transition greenstone. 
5. Granite, Sienite, and Porphyry. —These have the 
same composition as in the primitive class; and, independent 
of the characters derived from their mass, and their particular 
imbedded minerals and veins, are distinguished by the grey- 
waeke, with which they are associated. 
6. Gneiss, and Mica Slate. —These rocks occasionally 
occur associated with the greywacke and other members of 
this class. 
7. Serpentine and Quartz Rock. —These very nearly re¬ 
semble those of the primitive class, but are distinguished from 
them by their connexion with greywacke, &c. 
The extensive and very" interesting class of rocks termed 
secondary, rests immediately on those of the transition class; 
Vol. XXII. No. 1491. 
* 
C K. 
but when these are wanting, it rests on primitive rocks; and 
when the rocks of the two preceding classes occurin the same 
district with the secondary, the two former generally occupy 
a higher level. The hills of secondary districts are lower, 
rounder, with gentler acclivities, and fewer cliffs than those in 
transition districts; and their valleys are shallower, and their 
bottoms less inclined. Nearly all the secondary formations 
are more or less distinctly stratified, and the strata are more 
frequently horizontal, or slightly inclined, than in the older 
rocks. That regularity of direction of the strata, so promi¬ 
nent in the two preceding classes, has not, been observed in 
the present. 
Many of the secondary rocks, from their conglomerated 
structure, present a mechanical and not chemical aspect, and 
even the limestones of this series approach more nearly to 
the mechanical formation, than is observed in those of the 
preceding classes. 
Secondary rocks are further particularly distinguished by 
the great variety and abundance of fossil organic remains 
which they contain. These extend throughout the whole 
secondary series, abounding in some formations, as limestone, 
and in others as gypsum and trap, appearing rarely, and in 
small quantities. In the older formations, fossil remains of 
oviparous quadrupeds or lizards are met with, while, in the 
newer members of the series, remains of true quadrupeds, as 
of opossums, occur; and Werner, long ago, pointed out 
among the secondary formations the gradual rise of the ani¬ 
mals in the zoological scale, according to the date of the for¬ 
mation in which their remains are found, viz. that in the old¬ 
est secondary rocks, the animal remains were of tribes lower 
in organization than those met with in formations in the 
middle of the series, and that those found in the newest 
members of the class were of animals much more perfect than 
those in the middle part of the series. 
Coal, which we have already enumerated in the primitive 
and transition classes, occurs in great abundance in the secon¬ 
dary class, and, besides the glance-coal, the only kind of 
coal found in the formations older than the secondary class, 
contains also the black or bituminous coal, (which has much 
more the aspect of a vegetable formation than the glance- 
coal,) and brown coal, a mineral of undoubted vegetable 
origin. 
Secondary rocks are much less metalliferous than the tran¬ 
sition and primitive, and hitherto the principal repositories, 
of ore have been met with in the lower parts of the series, 
viz. in the mountain limestone, lower part of the coal forma¬ 
tion, magnesian limestone, and in the lower part of the new 
red sandstone. The most abundant metals are iron, lead, 
and copper, and to these, as a metal of rather abundant oc¬ 
currence, may be added zinc, in the form of calamine, mer¬ 
cury in form of cinnabar, and cobalt. It is in the secondary 
class of rocks, that rock-salt first makes its appearance in 
quantity, and in the form of imbedded masses, and beds 
associated with gypsum and saliniferous clay. 
The gems, as we have already remarked, are almost en¬ 
tirely confined to the primitive class, being of comparatively 
rare occurrence in transition rocks, and still less frequent in 
the rocks of this class. 
The principal secondary rocks are sandstone, limestone, 
and trap, and these are arranged in various positions, and 
associated with other rocks. 
We shall now enumerate them in the order of their rela¬ 
tive position, 
1. First Sandstone, or Old Red Sandstone Formation.— 
This is a reddish-brown sandstone, principally composed 
of particles of quartz, either without ground, or connected 
together by a basis or ground of iron-shot clay. It passes 
into greywacke, as on the coast of Galloway. It rests upon 
the rocks of the transition class. 
2. First Secondary Limestone, or Mountain Limestone— 
is a compact blueish-grey limestone, full of encrinites, corals, 
and shells; often contains caverns, and sometimes alternates 
with the sandstone, slate-clay, and other rocks of the coal 
formation. It lies immediately on the old red sandstone. 
3. Coal Formation.—'This is an alternation of grey and 
2 T white 
