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Subaerial Applied to phenomena which take place on the earth's surface, in 

 contradistinction to those which take place under water, these being 

 termed subaqueous. 



Subaqueous Applied to rocks deposited under water. 



Subsequent A word used by some authors synonymously with intrusive (q.v.). 



Synchronous Of the same age. 



Synelase See Joint. 



Synclinal A term applied to strata which dip towards a central axis. See 

 Anticlinal. 



Tabular Structure The bedded structure observed in many igneous rocks, and 

 caused by well-defined horizontal joints. 



Talcose Of the nature of talc, or containing a large proportion of that 

 mineral. 



Tautozonal The name given to the faces of crystals which lie in one and the 

 same zone. 



Tetartohedral The term applied to crystals which present only a quarter of 

 the full number of faces, the others having disappeared in accordance with 

 some law of symmetry. 



Tetragonal The name given to one of the six systems of crystals. It is 

 characterized by five planes of symmetry. The forms of the tetragonal 

 system are referred to three axes standing at right angles to one another, 

 two of which are equal and similar, the other (the chief axis) unequal 

 and dissimilar. Syn. Quadratic, Pyramidal, Prismatic, Dimetric, Monodi- 

 metric, Zwei-und-einaxiges. 



Tetrahedron The hemihedral form derived from the regular octahedron by the 

 disappearance of its alternate faces. The figure which thus results is 

 enclosed by four equilateral triangles. Since there are two different sets 

 of faces, either of which may be made to vanish, two different tetrahedra 

 are possible and occur in nature. These are distinguished from one 

 another by the one being termed positive, the other negative. 



Tetrakishexahedron The tetrakishexahedra are a series of forms in the 

 regular system contained by twenty-four isoceles triangles, the limits of 

 which are formed by the cube and the rhombic dodecahedron respectively. 

 Each face cuts one axis at unit distance, one at a distance equal to a 

 rational quantity, m, and is parallel to the third. Syn. Pyramidal Cube. 



Texture The French are inclined to apply the word structure to the nature of 

 the form assumed by a rock on consolidation ; whereas the word texture 

 is applied to the size and mutual arrangement of the constituents of the 

 rock. In Germany and England, however, these words are used more or 

 less indifferently. 



Thrust-plane Term applied to a reversed fault which is only slightly inclined 

 to the horizon. 



Thalassic A term applied to sedimentary rocks which are laid down on the 

 sea-bottom. 



Trachytic A structure characteristic of volcanic rocks, in which the minerals 

 occur in lath-shaped crystals and microlites, frequently showing fluxion- 

 structure. The name was originally given to these rocks on account of 

 their rough texture. Syn. Trachytoid, Pilotaxitic. 



