ROCK. 
■wards ■which the stratum inclines. The di- 
rection is the angle which the stratum 
makes with the meridian, and is deter- 
mined by the compass. It is always at 
right angles to the dip. 
The primitive rocks are chiefly composed 
of substances, which chiefly consist of the 
siliceous and argillaceous earths. 1. Gra- 
nite, the moor-stone of Cornwall, is a gra- 
nular rock composed of felspar, mica, and 
quartz, united in various proportions. — 
Schorl, garnet, tin-stone, adularia, chlorite, 
and rock crystal, are among the accidental 
minerals which occur in this rock : it some- 
times exists in large distinct globular, and 
sometimes in columnar concretions: it is 
sometimes stratified, but seldom contains 
any foreign beds. 2. Gneiss, is a strati- 
fied rock, formed of the same component 
parts as granite, but the mica exists in 
larger proportion than in granite : it some- 
times contains schorl, and, but more rarely, 
garnet and hornblende ; its sti-ucture passes 
from that which approaches to the granu- 
lar structure of granite, to the undulated, 
and even the slaty structure. It is very 
frequently metalliferous, there being few 
metals which are not found in it. 3. Mica 
slate is likewise a distinctly stratified rock, 
which rests on gneiss : it is composed of 
mica and quartz, disposed in a slaty struc- 
ture: it frequently contains garnets, and 
sometimes hornblende, schorl, and tour- 
maline, kyanite, rutile, and felspar. Like 
gneiss it is frequently metalliferous, the ores 
generally occurring in beds ; whereas in 
gneiss the ore is most frequently found in 
veins. 4. Clay-slate is a simple rock, and 
follows the foregoing in the series of primi- 
tive rocks : it sometimes contains schorl, 
tourmaline, garnet, hornblende, chiasto- 
Kte, and aciynolite. There appear to be 
four different kinds of clay-slate, chiefly dis- 
tinguishable by their colours: yellowish 
grey which connects clay-slate with mica 
slate ; dark and bluish grey, used as roof- 
slate ; greenish grey, and lastly bluish and 
reddish grey, containing a few scales of 
mica. The rocks peculiar to this formation 
are whet-slate, roof-slate, chlorite-slate, 
talc-slate, alum-slate, drawing-slate, pot- 
stone, and flinty-slate. This, like those 
already mentioned, is a widely extended 
rock, and is also one of the most metalli- 
ferous. 5. Primitive lime-stone, is a sim- 
ple mountain rock, which is more or less 
distinctly stratified, and is frequently me- 
talliferous ; its colours are various, and its 
structure is always granular. Quartz and 
mica frequently occur in it accidentally : it 
also sometimes contains hornblende, acty- 
nolite, asbestos, serpentine, talc, steatite, 
tremolite, garnet, calcareous spar, and 
slate spar. 6. Primitive trap, is a moun- 
tain which seems intimately connected with 
clay-slate. The terra trap had been long 
used without a definite signification; but 
W erner has restricted its application to rocks, 
principally containing hornblende, and black 
iron-clay; the iron-clay first appearing in 
the transition, and increasing in rocks of the 
newer periods. There are three distinct 
species of primitive trap. Common horn- 
blende rock, under which are comprised, 
hornblende rock, and hornblende slate. 
Hornblende with felspar, a subordinate 
kind of which is greenstone, which has 
the following varieties : common green- 
stone, a granular aggregate of hornblende 
and felspar. Porphyritic greenstone is the 
former, containing crystals of felspar. 
Greenstone porphyry, is the black por- 
phyry of the ancients; crystals of felspar 
are here also included, but, the granular 
structure of the basis is hardly discoverable. 
Green porphyry, in which the granular 
structure is no longer visible, and crystals 
of compact felspar are included. Tiie se- 
cond species of primitive trap is greenstone 
slate, composed of hornblende and compact 
felspar, arranged in a slaty structure ; and 
the third is an intimate mixture of horn- 
blende with felspar, including mica in scales. 
It is found in beds in gneiss and mica- 
slate. 7. Serpentine is a simple mountain 
rock, indistinctly stratified. A great variety 
of other mineral bodies are found in it, and 
it is' sometimes indeterminately mixed with 
limestone, forming what is termed verde 
antico. 8. Porphyry is a compound rock 
formed of one substance in the form of 
grains or crystals, imbedded in another as 
its basis. The base is clay-stone, horn- 
stone, compact felspar, pitch-stone, pearl- 
stone, or obsidian : the imbedded crystals 
are of quartz or felspar. There appears to 
be two formations of porphyry ; the oldest 
consists principally of hornstone and felspar 
porphyry; and the newer of clay, pitch- 
stone, pearl-stone, and obsidian porphyry. 
9. Sienite is a compound, granular, aggre- 
gated rock, formed of felspar and horn- 
blende, and sometimes containing quartz 
and black mica. The hornblende distin- 
guishes this rock from granite ; but the fel- 
spar, which is almost always red, and sel- 
