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Group C.—Rocks Partly Mechanically, Partly Chemically.
AND PARTLY ORGANICALLY FORMED.
Subgroup A. Laminated, or stratified coal.
Subvar. a. Parrot coal.
b. Horn coal.
b. Splint coal.
d. Bituminous shale.
e. Carbonaceous shale.
Subvar. a. Batt, or bass.
e. Slaty culm.
Var. a. Compact limestone.
Subvar. a. Pisolyte.
b. Dolomitic oolyte.
Subvar. a. Indurated chalk.
b. Chalk rock.
c. Pisolitic chalk.
d. Glauconitic chalk.
e. Ferruginous chalk. /. Calcsinter.
Subvar. a. Travertine.
d. Roekmeal. g. Coral-reef limestone.
h. Brecciated limestone.
i. Limestone conglomerate.
j. Bubbly limestone.
I. Siliceous limestone,
Var. p. Ferruginous.
Subgroup C. Dolomyte.
Var. a. Granular.
D. Marine drift.
(See Class II., Subgroup B, Var. e.)
Order l.—INOENITE ROCKS.
THE typical rock of this class is a crystalline aggregate of quartz, felspars, and micas, nearly universally known as granite or granyte [Celtic gran, Lat.granum, a grain.]* There are, however, rocks called granite in which part of the mica is replaced by other minerals, sometimes to such an extent that the mica is only microscopically visible. Granite usually is a quaternary or quinary aggregate, containing, along with quartz, one or two felspars and one or two micas; nevertheless, other minerals are often present, more especially pyrite and marcasite.f Some granites are undoubtedly intrusive, while others appear to be only in part intrusive, portions having been formed in situ; that is, having relations as to position with the associated rocks similar to those which now exist. Typical granites weather with a peculiar rough, rugged aspect.
* In Cornwall granite formerly was called growan, from gronen, a grain.
+ Some of the granites that weather or disintegrate freely appear to have marcasite as an ingredient disseminated in minute grains throughout the mass.
A. Intrusive Granite; Highly Siliceous Granite;
Leinster Granite; Oughterard Granite [intruded into the place it now occupies]. — An aggregate of quartz and orthoclase with black and white mica; pyrite and marcasite are often constituents; while Haughton seems to believe that most, if not all, intrusive granites also contain albite.
Note.—Haughton finds the intrusive granites of Cornwall, Devon, Leinster, and Ulster, to contain "quartz, orthoclase, margarodite, and lepidomelane." He has also detected albite.
Varieties in Composition.
A. Pyritous Intrusive, or Highly Siliceous Gra
Nite. — When pyrite or marcasite is a component, galenite and chalcopyrite often occur as accessories. a. Beresyte, Beresite.—Pyritous highly siliceous granite containing gold.
B. Albitic Granite (Dana).— Containing albite as
well as orthoclase. G. Schorlaceous Granite. — With tourmaline (schorl) in addition to the mica.
In the highly siliceous or intrusive granite, as also in all granite, and some of the other ingenite rocks, are veins of segregation, which in the granites generally form two distinct varieties. One kind appears as irregular veins, nests, or patches, that have no deep-seated source, but die out every way, often in very short distances. The other variety makes regular, often dyke-like, veins, from half an inch or less in thickness, to about two or three yards in width. These latter appear to have segregated from the deep-seated fluid or semi-fluid portions of the mass, and to have been forced up into