96 



STEUCTUKAL GEOLOGY. 



(18) Scratches (striae) or furrows or polished surfaces sometimes cover 

 rocks, which have been produced by abrasion attending niovements. They 

 often cover the walls of fissures, and sometimes the surfaces of beds of rock ; 

 and in such cases they are called by the miners' term, slickensides. They 

 occur also over the rocky surface of a country as a result of past or recent 

 glacier flows; and such are called simply glacier scratches or striae. This 

 subject is further explained under Dynamical Geology. 



(19) Concentric structure. — In concentric structure there is an aggre- 

 gation of matter around a center, making, usually, spheres or flattened 

 spheroids, as in Figs. 69-83. The form is usually dependent on growth by 

 deposition from a solution around a center, so that the growth is outward, 

 or centrifugal. In ordinary concretions it is growth by accretion, and it 

 sometimes produces a series of distinct concentric layers. The forms are 



69-80. 



spherical (Fig. 69) ; more frequently flattened spheroids (Figs. 74, 83) ; and 

 very frequently aggregations of concretions that are symmetrical in arrange- 

 ment (Figs. 79, 80). Concentric layers are shown in Figs. 71 and 81. At 

 the center there may be, as a nucleus, a shell (Fig. 70), or a spider, or insect, 

 or leaf, or merely a grain of sand undistinguishable by the unaided eye. They 

 often form as the first step in the process of consolidation, and make a rock 

 consisting of concretions which may disappear when the consolidation is com- 

 plete. Some layers may have spherical concretions, and another above and 

 below flattened (Fig. 82), those beds in which filtrating waters spread with 

 equal facility in all directions having spherical, and those of a laminated 

 structure, in which the waters spread laterally most easily, having spheroidal 

 or flattened kinds. They are sometimes hollow rings, or contain a ball within 

 (Figs. 77,78). 



The kind represented in Fig. 81, in which the concretions are about as 

 large as peas, is called pisolite, from the Latin for pea. A similar kind, 

 having the spheres about as large as the roe of fish, but not often with con- 

 centric layers, is the rock oolyte. Oolyte is now forming on the Florida 



