TEREANES. 



99 



(2) The depositions of a moiintain stream where it abruptly reaches a 

 plain make a broad low cone, stratified parallel to its surface, called an 

 "alluvial cone." See page 194, under Rivers. Such fresh-water accumula- 

 tions have thus far been found only among recent formations. 



(3) The deposits of sand constituting a sea-beach, as stated on page 93^ 

 take the slope of the beach, which may vary from 3° and less to 18° ; and 

 they have distinct bedding parallel to the sloping surface. 



These cases of an inclined position are relatively of limited extent. 

 They do not affect the truth of the general proposition that the original 

 position of the earth's great stratified rocks is essentially horizontal. 



(4) Another example of inclined stratification is afforde.d by the volcanic 

 mountains of the globe, whose lava-streams usually have a pitch between 

 3° and 20°, but may have a less or a much greater pitch. In the volcanic 

 mountain the stratification is pericentric, more completely so than in the 

 alluvial cone. 



3. Fractured and Displaced Strata. 



Strata, however continuous and horizontal when first formed, are, at the 

 present time, more or less divided up by planes of fracture, and sometimes 

 profoundly so. In general, also, they have lost their original horizontality, 

 and instead the beds have a pitch, small or large, sometimes rising to verti- 

 cality or even beyond. In many mountain regions the strata are in great 

 flexures, each flexure miles in sweep. Further, fractured and flexed strata 

 have often been displaced along a fracture, either upward or downward, in 

 some cases a few inches, in others miles, the rocks on one side of the plane 

 of fracture being dropped down or shoved up to this extent. In addition, 

 all regions, especially mountain regions, have lost a vast amount of rock 

 through the long-continued wear of flowing waters, which has reduced flex- 

 ures to ledges and level surfaces, concealing displacements and disguising 

 greatly the original features of a region. 



The following are explanations of terms used in describing upturned and 

 displaced rocks : — 



An outcrop is a projecting ledge of rock (Fig. 87). 



87. 



The dip is the angle which the beds make with a horizontal surface ; and 

 its direction is down the sloping surface, in the direction in which the angle is 

 greatest — dp in Figs. 87 and 88. The inclination of a sloping bed or of a Avail 



