578 
ELEMENTS OF GEOLOGY. 
stratified and foliated rocks having the same component ma¬ 
terials as granite or syenite, but also in a wider sense to em¬ 
brace the formation with which other members of the meta- 
morphic series, such as hornblende-schist, may alternate, and 
which are then considered subordinate to the true gneiss. 
The different varieties of rock allied to gneiss, into which 
feldspar enters as an essential ingredient, will be understood 
by referring to what was said of granite. Thus, for example, 
hornblende may be superadded to mica, quartz, and feldspar, 
forming a hornblendic or syenitic gneiss; or talc may be 
substituted for mica, constituting talcose gneiss (called strat¬ 
ified protogine by the French), a rock composed of feldspar, 
quartz, and talc, in distinct crystals or grains. 
Eurite^ which has already been mentioned as a plutonic 
rock, occurs also with precisely the same composition in beds 
subordinate to gneiss or mica-slate. 
Hornblende-schist is usually black, and composed princi¬ 
pally of hornblende, with a variable quantity of feldspar, and 
sometimes grains of quartz. When the hornblende and feld¬ 
spar are in nearly equal quantities, and the rock is not slaty, 
it corresponds in character with the greenstones of the trap 
family, and has been called “ primitive greenstone.” It may 
be termed hornblende rock, or amphibolite. Some of these 
hornblendic masses may really have been volcanic rocks, 
which have since assumed a more crystalline or metamorphic 
texture. 
Serpentine is a greenish rock, a silicate of magnesia, in 
which there is sometimes from 30 to 40 per cent, of magne¬ 
sia. It enters largely into the composition of a trap dike cut¬ 
ting through Old Red Sandstone in Forfarshire, and in that 
case is probably an altered basaltic dike which had contained 
much olivine. The theory of its having been originally a 
volcanic product subsequently altered by metamorphism 
may at first sight seem inconsistent with its occurrence in 
large and regularly stratified masses in the metamorphic se¬ 
ries in Scotland, as in Aberdeenshire. But it has been sug¬ 
gested in explanation that such serpentine may have been 
originally regularly-bedded trap tuff, and volcanic breccia, 
with much olivine, which would still retain a stratified ap¬ 
pearance after their conversion into a metamorphic rock. 
Actinolite Schist is a slaty foliated rock, composed chiefly 
of actinolite, an emerald-green mineral, allied to hornblende, 
with some admixture of garnet, mica, and quartz. 
Mica-schist or Micaceous Schist^ is, next to gneiss, one of 
the most abundant rocks of the metamorphic series. It is 
slaty, essentially composed of mica and quartz, the mica 
