448 



Spheric Structure (Fr. structure globuleuse ; Ger. spharische or kuglige 

 Structur) An arrangement of the constituent particles of a rock around 

 certain centres so as to produce radiate, concentric, or granular spheroidal 

 aggregates. It embraces the following structural varieties : oolitic struc- 

 ture, variolitic structure, centric structure, spherulitic (felsospheric) struc- 

 ture ; further, globospheric, belonospheric, and granospheric structure (q.v.}. 



Spherocrystal A term applied to those belonospherites consisting of only one 

 mineral in radiating fibres or needles. 



Spheroid The name given to the ball-shaped concretionary bodies occurring in 

 certain orbicular granites and granitic rocks (e.g., the Napoleonite of Corsica). 



Spheroidal Structure The name given to the property possessed by certain 

 igneous rocks (basalt, phonolite, &c.) of weathering into ball-shaped 

 masses, each of which is usually made up of concentric layers. This 

 phenomenon is connected with the contraction undergone by the rock 

 during cooling. 



Spherulitic Structure (Fr, texture globulaire) Peculiar to vitreous rocks, and 

 so called because of the small spherular bodies (itpherulites) they contain. 

 The latter, viewed in thin section under a microscope, are seen to possess 

 a divergent fibrous structure, and show, under crossed nicols, the 

 characteristic black cross. 



Spicule A small dart-shaped body; a term sometimes applied to minerals 

 occurring in that form. 



Stellate (Ger. sternfb'rmig) Kadiating from a centre, as some fibrous minerals 

 (e.g., actinolite). 



Strain-Slip Cleavage See Ausweichungsclivage. 



Stratification foliation Foliation in which the minerals are arranged parallel 

 to the stratification of the rock. 



Stratigraphy (Fr, and Ger. Stratigraphie) That department of geology that 

 treats of the relative disposition of strata, and of the disturbances they 

 have undergone subsequent to deposition. 



Stratum (Fr. couche, strate ; Ger. Schicht) The name given to the layers 

 (strata) in which aqueous rocks are deposited. It is the lowest subdivision 

 in the classification recommended by the International Geological Congress 

 at the Bologna meeting (1881). According to this scheme 



Strata unite to form an assize, or beds; 



Assizes to form a stage (Fr. (Stage ; Ger. Stufe) ; 



Stages to form a series, or section (Fr. series ; Ger. Abtheilung) ; 



Series to form a system (Fr. terrain ; Ger. System) ; 



Systems to form a group (Fr. groupe ; Ger. Gruppe). 



Streckung (Ger.) The German expression for the extension of the com- 

 ponents of a rock along parallel lines, whether it be due to differential 

 movements in the molten magma or to the mechanical deformation of the 

 solid rock. Syn. Linear-parallel structur. 



Striated (Fr. strie ; Ger. gestreift) Marked with fine parallel straight lines, as the 

 triclinic felspars ; applied also to rocks which have been scored by ice action. 



Strike The strike of a bed is the direction (expressed by reference to the points 

 of the compass) of the line formed by the intersection of the plane of the 

 bed with the plane of the horizon. 



StylolitCS A curious structure found in certain limestones, dolomites, and 

 marls. It consists of an arrangement of irregular cylindrical bodies 

 which stand at right angles to stratification and are often abruptly 

 terminated. 



