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Stilbite. A crystallized simple mineral, usually white, one of the 

 Zeolite family, frequently included in the mass of the trap 

 rocks. 



Strata, Stratum. When several rocks lie like the leaves of a 

 book, one upon another, each individual forms a stratum ; 

 — strata is the plural of the word. Etym., stratum, part of a 

 Latin verb signifying to strew or lay out. 



Strike. The direction or line of bearing of strata, which is always 

 at right angles to their prevailing dip. 



Syenite. A kind of granite, so called because it was brought from 

 Syene in Egypt. 



Synclinal Axis. When the strata dip in opposite directions 

 towards a common central imaginary line, it is called a syn- 

 clinal line or axis. 



Talus. When fragments are broken off by the action of the weather 

 from the face of a steep rock, as they accumulate at its foot, 

 they form a sloping heap, called a talus. The term is borrowed 

 from the language of fortification, where talus means the out- 

 side of a wall of which the thickness is diminished by degrees, 

 as it rises in height, to make it the firmer. 



Tertiary Strata. A series of sedimentary rocks, with characters 

 which distinguish them from two other great series of strata, — 

 the secondary and primary, which lie beneath them. 



Testacea. Molluscous animals, having a shelly covering. Etym., 

 testa, a shell, such as snails, whelks, oysters, &c. 



Thin out. When a stratum, in the course of its prolongation in 

 any direction, becomes gradually less in thickness, the two 

 surfaces approach nearer and nearer ; and when at last they 

 meet, the stratum is said to thin out, or disappear. 



Trap and Trappean Rocks. Volcanic rocks composed of felspar, 

 augite, and hornblende. The various proportions and state of 

 aggregation of these simple minerals, and differences in exter- 

 nal forms, give rise to varieties, which have received distinct 

 appellations, such as basalt, amygdaloid, dolorite, greenstone, 

 and others. The term is derived from trappa, a Swedish word 

 for stair, because the rocks of this class often occur in large 

 tabular masses, rising one above another, like the steps of a 

 staircase. 



