1830.] 
45 
and Mineralogical Observations. 
glassy felspar forming a great portion of the mass, and they melt into a white glass. 
Trachyte is, therefore, merely a variety of lava, excepting as to the date of formation, 
and there is a transition through all the above trap and trachyte rocks, and from 
them intosienite and porphyry. 
Meteoric Stones are, many of them, very similar to varieties of greenstone, but 
mixed with iron ; others are composed of iron nearly pure or native, so as to be mal- 
leable ; sometimes the iron is in so great a proportion as to form almost the mass. 
The metal nickel is always found in meteoric iron, and is considered to give the 
property of resisting oxidation in the atmosphere. 
Wacke appears to be a basalt half decomposed, or semi-indurated, or a hard 
ferruginous clay ; it is sometimes ainygdaloidal. 
Porphyry is composed of crystals, embedded in and disseminated through a com- 
pact mass, which is called the base or paste. These crystals are of cotemporaneous 
formation with the base or mass, since they have their angles and faces of crystal- 
lization perfect. These crystals are of felspar, and the base may be considered a 
compact granite or greenstone, felspar being the predominating ingredient. Suck 
are the rocks to which the name porphyry is particularly given : but any rock hav- 
ing crystals so disseminated through it, is a porphyry, and may be named according 
to its composition, as porphyritic claystoue, obsidian, &c. The base of the porphyry 
just mentioned, is the Eurite of the French ; and when hornblende predominates, it 
is their Ophvnite. 
Amygdaloid has the same, or nearly the same composition as the paste forming 
the abovementioned rock, but instead of crystals disseminated through its mass, as 
in porphyry, it has round or elongated nodules, generally of calcareous matter, which 
appear to have been formed in cavities of the above figures previously existing by 
water, containing lime in solution, percolating through the rock ; when these cavi- 
ties are not tilled, they have frequently crystals of calcareous spar or zeolite im- 
planted on their sides. 
Serpentine is chiefly composed of silica and magnesia ; it is compact, but varie- 
gated in its colour ; it is the matrix of many magnesian minerals, of asbestos, ecume 
tie mer, &c. The green colour is given by the protoxide of iron, and the chromate 
of iron. It may be remarked, that the different oxides of iron appear to give the 
colouring matter to nearly all the minerals, rocks, and clays. 
Diallage Rock is composed of diallage, or diallage and felspar ; it is the Schil- 
lerfels of the Germans, and is found with beds of gneis and other rocks of the 
primitive series. 
Quartz Rock is quartz mixed generally with a little mica; it is the Gneisen of 
the Germans: their W eisen is felspar and mica; therefore, granite without the quartz. 
^ Pegmatite is composed of quartz and felspar, or granite without the mica* 
Flints and chalcedony are pure silex, and are merely varieties of quartz. Siliceous 
tufa is the deposit from water containing silex in solution. The Geiser fountain in 
Iceland appears to be almost the only deposit of this nature now in action : the mi- 
neral formed has less specific gravity than quartz, and comes nearer to obsidian or 
the opal. 
Sandstones are considered to be formed from the debris of former rocks ; the 
particles are commonly quartz, which may be supposed to have resisted decompo- 
sition more than the other minerals of which the rock was formed as granite. These 
grains are united into one mass by a cement of silex, argil, or lime ; the cement of 
lime will be known by its effervescing with acids ; that of argile is less hard than 
the siliceous, but these are often mixed in the same. 
Pudding Stone, or conglomerate, is of the same composition as sandstone, 
only the parts are larger and always water-worn or rounded. 
Breccia is also of the same composition as sandstone, but differing from the con- 
glomerate in being formed of angular fragments of rocks, as its name indicates, 
instead of water-worn or rounded pebbles cemented together. 
Grauwacke' is a sort of breccia or sandstone, containing fragments of primitive 
slate, and some organic impressions ; it is the oldest sandstone in the series ; it lies 
under the coal formation, and is, therefore, in the transition class ; it lias a siliceous 
or argillaceous cement. It corresponds with the beds of old red sandstone of 
the English geologists. 
Grauwacke Slate is a variety of the above which has passed into a schistose 
structure. Greenstone slate is similarly connected with greenstone, and in the 
same manner. By some geologists gneiss has been termed schistose granite. 
Sandstone Slates are in similar relation to the different beds of sandstone. 
Among the sandstones, particularly in the lower portion of the series, there are 
