STRATIFIED ROCKS. 



193 



ways represents a yap in the record a lost interval of 

 time which may be very long, viz., the whole time dur- 

 ing which the erosion was going on. 



FIG. 109. In all : , land ; , sea-level ; fih, shore-line ; 56, sea-bottom ; Sett 



sediments. 



A group of conformable strata usually form a geologi- 

 cal formation, and a line of unconformity usually sepa- 

 rates two different geological formations. The division 

 of the strata into formations, however, is based also on 

 other characters, viz., the contained fossils. The subject 

 will be taken up again under that head. 



Cleavage Structure. 



Stratification is an original structure, i. e., impressed' 

 at the time of deposit of sediments. Cleavage is a super- 

 induced or subsequent structure, but it so simulates 



LE CONTE, GEOL. 13 



