174 



STRATIFIED OR SEDIMENTARY ROCKS 



of unvarying thickness, and laid atop of each other like the sheets of a 

 ream of paper. On the contrary, each stratum, when first deposited, 

 must be regarded as a widely-expanded cake, thickest in the middle and 

 thinning out at the edges, and interlapping there with other similar 

 cakes. Fig. 138 is a diagram showing the mode of interlapping. The 



Fig. 138.— Diagram showing Thinning out of Beds: a, sandstones and conglomerates; b, limestones. 



extent of these cakes depends upon the nature of the material. In fine 

 materials strata assume the form of extensive thin sheets, while coarse 

 materials thin out more rapidly, and are therefore more local. 



The most important apparent exception to the law of original hori- 

 zontality is the phenomenon of oblique or cross lamination. This kind 



of lamination is formed by rapid, shift- 

 ing currents, bearing abundance of 

 coarse materials, or by chafing of waves 

 on an exposed beach. Many examples 

 of similar lamination are found in 

 rocks of previous epochs. Figs. 139 

 and 140 represent such examples. In 

 some cases oblique lamination may be 

 mistaken for highly-inclined strata ; careful examination, however, will 

 show that the strata are not parallel with the laminae. The strata were 



Fig. 139.— Oblique Lamination. 



Fig. 140.— Section on Mississippi Central Railroad at Oxford (after Hilgard): Oblique Lamination. 



originally (and in the cases represented in the figures are still) hori- 

 zontal, while the laminae are oblique. 



Elevated, Inclined, and Folded Strata. — We may assume with con- 

 fidence that stratified rocks were deposited as sediments at the bottom 

 of water and in a horizontal or nearly horizontal position. But we do not 

 now find them usually in this condition, place, or position. Sometimes, 

 indeed, they are still soft, but usually stony ; sometimes in the vicinity 



