GLOSSARY OF GEOLOGICAL AND SLATE-QUARRY TERMS, 
As this bulletin is intended primarily for the use of persons unfamiliar with geolog-i 
ical science, a number of the geological terms used in it, whose meaning is not self-* 
evident, are here explained, and for the benefit of geologists who may have occasion: 
to use the book some of the terms in use among slate quarrymen are also translated 
int< i scientific ones. 
Aggregate polarization. The optical property possessed by a thin section of slate cut transverse 
to its cleavage, which causes it, when viewed through a microscope under polarized light, to appear 
like a crystal of mica cut across its mineral cleavage. The aggregate polarization of a slate is due to 
the parallelism of the Hakes of mica in the slate, and this, when marked, shows it to be a mien-slate 
and not a day-date. A very brilliant aggregate polarization, coupled with regularity in size of parti- 
cles, indicates a high grade <>f lissility. 
Anticline. The arch part of a folded bed. 
Anticlinorium. a mountain mass arch-shaped jn its general internal structure. 
Authigenous. Minerals originating chemically within a rock are called authigenous. 
B \< k JOINT. Joint plane more or less parallel to the strike of the cleavage and frequently vertical. 
BED. A continuous mass of material deposited under water at about one time. 
Blindjoint. Obscure bedding plane. 
Bottom joint. Joint or bedding plane horizontal or nearly so. 
Breccia. Hock made up of angular fragments produced by crushingand then recemented by infil- 
trating mineral matter. 
Brecciated. Applied to a rock made up of angular fragments hut not transported. 
Clastic. Constituted of rocks or minerals which are fragments derived from other rocks. 
Cleave, slaty cleavage. 
Conformity. When two beds overlie in parallelism without any disturbance of the crust having 
affected the first one before the deposition of the second, they are said to be in conformity. 
Diagon \i. joints. Joints diagonal to the strike of the cleavage. 
Dip. The degree and the direction of the inclination of a bed, cleavage plane, joint, etc 
Dn 1 joint. Vertical joints about parallel to the direction of the cleavage dip. 
DIKE. Molten material erupted through a narrow fissure. 
End joint. Vertical joint about parallel to direction of the cleavage dip. 
Erosion. The " wear" of a rock surface by natural mechanical or chemical agencies. 
False cleavage. A secondary slip cleavage superinduced on slaty cleavage. (See p. 24 and Pis 
VI, XI, A.) 
Fault. A fracture resulting in a dislocation of the bedding or cleavage, one part sliding up 01 
down, or both changing positions along the fracture. 
Flints. A term applied alike to quartz veins or beds or quartzite. 
Formation. A larger group of beds possessing some common general characteristics or fossil forms 
differing from those of the beds above and below. 
Grain. An obscure vertical cleavage usually more or less parallel to the end or dip joints. 
Hards. Term applied to beds of quartzite or quartzitic slate. 
HCl. Hydrochloric or muriatic acid. 
Hogbacks. Shear zones. (See p. 29 and PI. VII.) 
Isoclinal. Folds with sides nearly parallel are said to be isoclinal. 
Matrix. Term used in microscopic descriptions of slate, etc., to denote the chief substance of tto 
slate itself, apart from the various mineral particles or crystals it may inclose. 
Mktamorphism. The process, partly physical, partly chemical, by which a rock is altered in tb 
molecular structure of its constituent minerals and frequently in the arrangement of its particles. 
Overlap. Where, owing to the depression of a coast and the consequent landward shifting of th 
shore line, the later marine sediments cover up the extremities of the older ones, there is said to bi 
an overlap. 
Pitch. The inclination of the axis of a fold of rock. 
146 
