Handbook of Paleontology 41 



in the sands and muds exposed by the tide are very apt 

 to be wiped out by the incoming tides and waves. 



Original structures include such features as bedding 

 or stratification, pinching or lensing out of strata, 

 lenses, progressive and regressive overlap, cross-bedding 

 etc. Stratification of sedimentary rocks is due to the 

 sorting power of the agent of transportation — wind or 

 running water. The stronger the current the heavier the 

 material carried. A slackening of current causes a depo- 

 sition of finer material upon coarser and vice versa, thus 

 giving layers or beds of different texture and also differ- 

 ent composition where another kind of material is sup- 

 plied. These beds or layers are known as strata (singu- 

 lar, stratum). Deposits have a uniform character, with- 

 out stratification, if the material is deposited under uni- 

 form conditions or the material supplied is of a uniform 

 character. Finer divisions of the strata are known as 

 laminae. Under quiet conditions, which, however, per- 

 mit sorting of material, very fine, even lamination is pro- 

 duced. Just as beds vary in texture from coarse to very 

 fine due to changes in carrying power of the transporting 

 agent, so there may be a variation from coarse below to 

 fine above in thin, uniform laminae deposited in- quiet 

 water, since even under the quietest conditions the larger 

 grains tend to settle out first. 



The dip of rock strata will be discussed under the sec- 

 tions on folding and other deformation. There is, how- 

 ever, a primary dip or original dip to be found in rock 

 layers. When sediments are deposited the strata assume 

 the attitude or slope of the underlying surface. This is 

 the primary dip, which is measured by the vertical angle 

 between this slope and the horizontal. 



The interval between any two beds in a sedimentary 



