447 



Scoriae Fragments of lava having a cindery aspect. 



Secondary (Ger. secundar, neugebildet) This word, as used in petrography, is 

 applied to minerals which are derived from the alteration of the original 

 constituents of a rock. 



Secondary Felspar -By the action of mechanical metamorphism, the original 

 felspars of a rock often undergo a regeneration the nature of which is not 

 yet understood. Under the microscope the remaining portion of the old 

 kaolinized felspar-grain is sometimes found surrounded by a layer of 

 fresh colourless granules, which in isolated cases show twin-striation. 

 Lessen regards the secondary felspar of many of the diabases of the Hartz 

 as albite. 



Secretionary A term used to express a growth from without inwards, in con- 

 tradistinction to concretionary, which signifies growth starting from a 

 central point or nucleus and proceeding outwards. 



Segregation- vein A vein which has been produced by the segregation of the 

 component mineral matter of a rock along fissures. 



Semicrystalline A term applied generally to eruptive rocks which are com- 

 posed partly of crystalline, partly of amorphous matter. This term 

 was also used by Naumann to designate clastic rocks in which there is 

 a predominance of a crystalline cement. 



Septaria The name given to concretionary nodules, often lenticular, occurring 

 in clays. Such nodules show, in the interior, cavities and cracks (septa), 

 which are empty or filled with secondary mineral substances. 



Shaly Exhibiting the fissile laminated character of a shale. 



Shearing Differential movement in a rock-mass. When the movement is 

 concentrated along a plane, this plane is said to be a shear-plane. 



Sheet (Fr. nappe ; Ger. Decke, intrusives Lager) Any eruptive mass which has 

 spread out over a large area, or has been intruded between strata so 

 as to appear at the surface as a bed. 



Siliceous Rich in silica, 



Sigmoidal Having the form of an 8. Slowly moving lava streams often 

 assume an internal sigmoidal structure, the folds produced by the unequal 

 movement of the different parts of the mass having this form. A similar 

 character is sometimes developed in foliated rocks which have been 

 subjected to dynamic metamorphism. See " Ausweichungsclivage." 



Skeleton-crystals (Ger. Krystallskelette, gestrickte Formen) The incipient 

 forms of crystallization in which some minerals, notably felspar, appear 

 when the consolidation of the rock has been rapid. See Fig. 7 of text. 



Slaty Having the cleavable structure of a slate ; applied also to fissile lime- 

 stones which admit of being split up into smooth slabs. 



Slickensides (Ger. Rutschflachen, Reibungsflachen, Scheifflachen, Spiegelflachen, 

 Harnische) The name given to the grooved and polished surfaces of rocks 

 produced by the friction of the walls of a fault or joint during dis- 

 placement. 



Solution-plane (Ger. Losnngsflache) The name proposed by von Ebner 

 (S. A. W., 1884, p. 368) for a plane in a crystal along which it is most 

 easily attacked when submitted to chemical action. 



Sphenoid The sphenoids are the hemihedral forms derived from the tetragonal 

 or rhombic protopyramids by the disappearance of alternate faces. Two 

 series of sphenoids, differing only in position, are thus obtained. These 

 are known respectively as the positive and negative series. The tetra- 

 gonal sphenoid resembles the tetrahedron, from which it differs only in 

 the inequality of its axes. 



