OVERLAP 



2^1 



case arises when the surface of the ground is made by cutting 

 down strata to the upper surface of a hard bed, which is then de- 

 pressed beneath the water, as a flat pavement, upon which new 

 material of a similar kind is laid down with hardly a perceptible 

 break. In the Rocky Mountain region remarkable instances of 

 this deceptive conformity occur, where, in the middle of a mass of 

 limestone apparently formed without any interruption, there is, in 

 reality, an enormous time-gap. Long and careful search has 

 made clear the nature of the contact and exposed the deception. 

 The lowest member of the upper series of strata in an uncon- 

 formity is very frequently a conglomerate or coarse sandstone, and 

 represents the beach formation of the sea advancing over the old 



FIG. 118. — Unconformity without change of dip, and overlap. 



land. These are called basal conglomerates. Such coarse beds 

 are, however, not always present, and they may be only locally 

 developed along a particular line. 



Unconformities may be confined to relatively restricted regions, 

 or they may extend over whole continents ; they are very useful 

 means of dividing the strata into natural chronological groups. 



Overlap. — When a series of strata is deposited in a basin with 

 sloping sides, or on one sloping side, each bed will extend farther 

 than the one upon which it lies, and thus in a thick mass of strata, 

 if the shelving bottom be gently inclined, the upper beds will ex- 

 tend far beyond the lower ones, or overlap them. (See Fig. 118.) 

 Overlap also occurs where the sea is advancing or transgressing 

 slowly across a subsiding land surface, the rate of depression not 

 much exceeding the rate of deposition. Here also each stratum 



