98 



STRUCTURAL GEOLOGY. 



by deposition from the same or some other mineral solution. Geodes are 

 common in veins of ore, and also occupying the cavities of amygdaloids. 



The concentric structure is produced also by consolidation 2^^'ogressing 

 inward from the exterior — a centripetal process. Spheroidal masses of sand, 

 often of oblong-spheroidal, as well as other shapes, colored deeply with iron 

 oxide, are often hard outside, and have mere loose sand within ; or they have 

 one or more concentric layers of ferruginous color within, or a series of 

 concentric shells of sand, and sometimes also a loose ball, as in Fig, 75. 



A concentric structure is produced also by decomposition along fracture- 

 planes, when these divide a rock into small portions (as explained on page 

 127), and also by alternate heating and cooling (page 337). 



I- 



2. Original Positions of Strata. 



Strata in their original positions are commonly horizontal, or nearly so. 

 The level plains of alluvium and the extensive delta and estuary flats 

 show the tendency in water to make its depositions in nearly horizontal 

 planes. The deposits formed over soundings along seacoasts are other 

 results of sea action ; and here the beds vary but little from horizontality. 

 Off the coast of New Jersey, for 80 miles out, the slope of the bottom 

 averages onl}'' 1 foot in 700, — which no eye could distinguish from a perfect 

 level. Over a considerable part of jSTew York and the States west and south- 

 west, and in many other regions of the globe, the strata are actually nearly 

 horizontal at the present time. In the Coal-formation, the strata of which 

 have a thickness, as has been stated, of 5000 to 15,000 feet, there is direct 

 proof that the beds were horizontal when formed ; for in many of the layers 

 there are fossil trees or stumps standing in the position of growth, and some- 

 times several of these rising from the same layer. 

 Fig. 85 represents these tilted coal-beds c, c, with the 

 stumps s, s, s. Since these trees must have grown in 

 a vertical position, like all others, and as now they 

 are actually at right angles to the layers, and parallel 

 to one another, they prove that the beds originally 

 were horizontal. The position of shell accumulations 

 and coral reefs in modern seas shows, further, that 

 all limestone strata must have been nearly or quite horizontal when they 

 were in the process of formation. 



Variations from horizontality. — (1) Some variation from horizontality 

 may be produced by the slope of 

 the sea-bottom in certain cases ; 

 and in lakes, off the mouths of 

 rivers (Fig. 86), quite a con- 

 siderable inclination may result 

 from the fact that the succes- 

 sive layers derived from the inflowing waters take the slope of the bottom 

 on which thev fall. ' 



85. 



86. 



