LITHOLOGICAL GEOLOGY.


and scoria usually take a stratified arrangement. A stratified ar-
rangement of igneous rocks is produced when the molten matter
issues from fissures at intervals in great quantities, and spreads out
in successive thin sheets over a large area. Such an accumulation
presents a very definite stratified appearance, viewed as a whole,
though of course each flow is in itself unstratified. The igneous
rocks of Lake Superior are a remarkable example of this.
Unstratyfed rocks are produced when great masses of molten
rock solidify.  They are probably also produced by an extreme
degree of metamorphism of what were originally stratified, sedi-
mentary rocks. Metamorphism, as usually known, does not entirely
destroy the bedded structure, though it may obliterate the finer
laminations. But it is highly probable that it sometimes, and per-
haps often, reaches a degree in which the main bedding lines are
destroyed also. The massive granites of the older formations which
lie at the base of the geological column may furnish examples of
this.
RocKs CONSIDERED WITH REFERENCE TO THEIR CHEMICAL NATURE.
It is often important to designate in general terms, the chemical
character of rocks. This is usually done by a class of adjective
terms, of which the following are some of the more common:
Silicious, those (1) in which silica is the predominant ingredient,
as meant when we say, a silicious rock; or (2) those in which it is a
notable though not predominant ingredient, as meant in the expres-
sion, a silicious limestone, or a silicious ore.
Calcareouas, those in which lime is (1) a predominant or (2) a char-
acteristic ingredient.
Ferrugi'nous, those in which iron (in the form of an oxide) is a
characteristic ingredient.
Carbonaceo as, those in which carbon is a more or less considerable
constituent.
Argllaceous, those consisting of, or characterized by clayey ma-
terial.
Another chemical distinction of growing importance is the dis-
crimination of rocks as acidic, neutral, and basic. Most rocks are
chemically of the nature of salts, i. e., they consist of a combination
of an acid ingredient and a basic or alkaline one. If these two
unite in such proportions as to satisfy mutually their chemical affini-
ties, the resultant rock is chemically rneatral. In case the acid in-
gredient predominates, the rock is termed acidic. In case the basic
is in excess, it is termed basic.


19.