202 GUIDEBOOK OF THE WESTERN UNITED STATES. 
Slate. A rock that by subjection to pressure within the earth has acquired the 
property of splitting smoothly into thin plates. The cleavage is smoother and 
more regular than the splitting of schist along its grain. 
Stratigraphy. The branch of geologic science that deals with the order and rela- 
tions of the strata of the earth’s crust. 
trike. The direction along which an inclined rock layer would meet the earth’s 
surface if that surface were level. The outcrop (which see) of a bed on a plain 
is coincident with its strike. 
Structure. In geology the forms assumed by sedimentary beds and igneous rocks 
that have been moved from their original position by forces within the earth or 
the forms taken by intrusive masses of igneous rock in connection with effects 
produced mechanically on neighboring rocks by the intrusion. Folds (anticlines 
and synclines) and faults are the principal mechanical effects considered under 
c Schistosity and cleavage are also structural features. 
cline. An inverted arch of bedded or layered rock suggestive in form of a canoe. 
(See fig. 28, p. 129.) 
Talus (pronounced tay’lus). The mass of loose rock fragments that accumulates at 
the base of a cliff or steep slope. (See Pl. XXII, B, p. 166 
Terrace. A steplike bench ona hillside. Most terraces along rivers are remnants 
of valley bottoms formed when the stream flowed at higher levels. Other ter- 
races have been formed by waves. Some terraces have been cut in solid rock, 
others have been built up of sand and gravel, and still others have been partly 
cut and partly built up. (See Pl. XX, p. 142.) 
Till. The deposit of mingled bowlders, rock fragments, and soil left behind by a 
melting glacier or deposited about its margin. 
Tuff. A rock consisting of a layer or layers of lava particles blown from a volcano. 
A fine tuff is often called volcanic ash and a coarse tuff is called breccia. 
Type locality. The place at which a formation is typically displayed and from 
which it is named; also the place at which a fossil or other geologic feature is 
displayed in typical form. 
Unconformity. A break in the regular succession of sedimentary rocks, indicated 
by the fact that one bed rests on the eroded surface of one or more beds which 
may have a distinctly different dip from the bed above. An unconformity may 
record. 
Vein. A mass of mineral material that has been deposited in or along a fissure in 
the rocks. A vein differs from a dike in that the vein material was introduced 
gradually by deposition from solution whereas a dike was intruded in a molten 
li t . 
on. 
Volcanic bomb. A rounded mass of lava thrown out while in a hot and pasty con- 
dition fromavolcano. A bomb, like a raindrop, is rounded in its passage through 
the air and may be covered with a cracked crust due to quick cooling. 
foleanic cone. A mountain or hill, usually of characteristic conical form, built up 
around a volcanic vent. The more nearly perfect cones are composed princi- 
pally of lava fragments and volcanic ashes, 
hee age Lava that has cooled and solidified before it has had time to crys- 
