GLOSSARY OF TERMS. 147 
Post. A mass of slate traversed by so many joints as to be useless. This term is also used todenote 
>ands of hard rock. 
Pseudomorph. A mineral that has assumed the crystal form of a different mineral us the result of 
he partial or entire alteration or replacement of the original mineral through chemical processes. 
Quartzite. A sandstone in which the grains are held together by a siliceous (quartz) cement. 
Ribbon. A line of bedding or a thin bed appearing on the cleavage surface, sometimes of a different 
:olor; or a small bed of quartzose or calcareous material either crossing or parallel to the cleavage. 
Vhen such ribbons are separated by beds of slate too thin to be worked, the ribbons and the small 
>eds are together designated as "ribbon." 
Sculping. Fracturing the slate along the grain, i. e., across the cleavage in the direction of the dip. 
Sericite. a ribbon-like or fibrous form of muscovite or potash mica. 
Shear zone. Hogback. (See p. 29 and PI. VII.) 
Slant. Longitudinal joint more or less parallel to cleavage and often slickensided. 
Slickensides. Surface of bed or joint plane along which the rock has slipped, polishing and groov- 
ng the surfaces. 
Slip. Occasional joint crossing the cleavage, hut of no great continuity. Slips are not infrequently 
ault planes. 
Slip cleavage. Microscopic folding and fracture, accompanied by slippage; quarrymen's "false 
:leavage." (See p. 24 and Pis. VI, XI, A.) 
Split. Slaty cleavage. 
Stratum. A bed. 
Stratification, Bedding, in distinction from cleavage. 
Strike. Direction at right angles to the inclination of a plane of bedding, cleavage, jointing, etc. 
Strike joint. Joint parallel to the strike of the cleavage. 
Sulphur. Iron pyrite. 
Syncline. The trough part of a fold of rock. 
Synclinorium. A mountain mass, in general internal structure trough shaped. 
Thick joint. Two or more parallel joints between which the slate has been broken up oi 
ecom posed. 
Till. A mixture of clay and pebbles deposited by glaciers. 
Top. The weathered surface of a slate mass or the shattered upper part of it. 
Unconformity. When the lower one of two contiguous deposits affords evidence of having been 
xposed to atmospheric erosion before the deposition of the upper one, there is said to be an uncon- 
ormity between them. 
Vein. When correctly used, denotes a more or less irregular, sometimes ramifying, mineral mass, 
ften of quartz, with calcite, etc., within the slate. Such veins are called veins of segregation, to 
how that they consist of matter collected from the adjacent rock by solvent waters. But, as gener- 
lly used by slate quarrymen, " vein " is the equivalent of bed or stratum. 
Wavers. Annelid trails. 
Wild rock. Any rock not fit for commercial slate. 
