44 
Directions for Geological 
[Fes, 
The mode in which these are found combined, and from that combination, or from 
the structure, give names to the different rocks, will be found in the works on geo- 
log}’, as also the transition which exists in one species of rock into another; from 
the absence of one or more of the composing minerals, or presence of others ; or 
from a change in the structure. This transition, as must be evident, frequently 
causes a doubt in the application of the names. 
To avoid, however, the necessitv of a constant reference to these works, the fol- 
lowing table, in which will be found described all the principal rocks, or at least 
those of common occurrence, has been made out. 
Some of the terms employed in it, primitive, transition, &c., refer to the place of 
the rocks or strata in the series, as to superposition, which will be afterwards ex- 
plained. 
Granite is composed of quartz, felspar and mica ; the proportions of each are 
various. Hornblende also sometimes enters into granite. 
Gneiss is composed of the same three minerals, but instead of being granular as 
granite, it has a schistose structure, from the plates of mica lying in the same di- 
rection. 
Mica Slate is composed of the same three minerals, or frequently of only mica 
and quartz, and has the same structure as gneiss, but the mica is in excess. 
Talc Slate and Chlorite Slate, arc similar to mica slate, the talc or chlorite, 
which is a green variety of talc, having taken the place of the mica. These two 
minerals are, however, often, with difficulty, distinguished from each other when 
disseminated in rocks. 
Clay Slate, (the slate used in roofing,) the best belongs to the primitive class, 
and appears to be composed of the same minerals as talc aud mica slate, but they 
are so intimately mixed that they cannot be detected by the eye. As the structure 
changes, a transition takes place from granite to gneiss, and from gneiss to the slates. 
Sienite is composed of hornblende and felspar, in well defined crystalline 
masses : quartz is also generally one of its component parts. It therefore differs 
from granite by the substitution of hornblende for mica ; but, as it has been seen, 
that granite sometimes contains crystals of hornblende, it will be evident there is a 
transition from one rock into the other. 
Greenstone or Whin, the Diabase of the French, is composed of hornblende 
and felspar, and sometimes a small portion of quartz ; the component minerals 
are generally so minute as to assimilate the appearance of the rock to a compact 
crystalline mass. When the greenstone is composed chiefly of hornblende, is high- 
ly crystalline, and is found alternating with the crystalline rocks of the primitive 
clm?s, it is generally termed hornblende ROf K. 
Basalt is composed of hornblende and felspar, but still more compact than the 
greenstone : some varieties contain augite, and large crystals of hornblende and 
augite are frequently imbedded in basalt : when these are in considerable number, it 
xs teimed porphyriHc, as will be afterwards explained. 
Obsidian and Pitchstone belong to this class, but their composition appears to 
'ary, and often to be chiefly silex. These rocks are generally found forming veins : 
they are vitreous or pitchlike ; their specific gravity is less than that of quartz ; their 
distinguishing characters will be found in the work of Mohs, as some of their varie- 
ties appear to be simple minerals. 
w a differs from basalt in composition, only from the prevalence of augite, and 
m structure from being generally porous, therefore, less dense ; but all the lavas, 
lien suthcmntly magnified, shew them to be composed of minute crystals, which ap- 
nvfn °e - G , s P ar a °d augite, or hornblende and crystals of leucite- Mica and 
eon«? ?• ,ron tre( l. u . entl y occur in lava. Pumice stone is a very porous lava, chiefly 
nartiHp« 1 '’f°H Vltn * ie d felspar or of quartz. Scoria and volcanic sand appear to be 
Quantit ° . le same nature as pumice ; and since volcanoes in action emit a great 
or sand ; n . r f Ut 1 , ,n , Htter in ixed with steam, which in its condensing unites the ashes 
tufa tint Hit. a S ° M mass > w hich is termed volcanic tufa. It is from the volcanic 
ori«in at Pnv^M Ca !* ct * Pozzalano is formed ; it is found in rocks of a volcanic 
similar rnrd.- !’ m * ie neighbourhood of Naples and neat Rome, and also from 
last the Diitfdi J uver £ ne * n France, and on the Rhine near Coblentz ; from which 
T K4p tcrmed Tn,ss > is obtained. 
French, ft 'K.ff™ f ° r greenstone ami basalts, AMPHIBOLITES of the 
Trachyte w iiw '? rU > e . nt * e > hy them named Amphibole , being the chief ingredient, 
canic origin but dlndr • w- 6 ”’ P articul arly by French geologists, to rocks of vol- 
Auvergne, Hune-arv . * “ om volcanoes no longer in action; such as are found m 
* a nd the Lower Rhine ; these are vitreous and cellular, white 
