24 PHYSIOGRAPHY [chap 



It is clear that when rain falls upon the surface A B, it 

 will be readily absorbed, at least if the sandy rock be dry, 

 and will gradually sink lower and lower until it reaches the 

 bottom of the upper bed C D. Here it comes in contact 

 with the surface of the clay, and, as the clay refuses to 

 absorb the water, its downward course is arrested. Should 

 the surface of the clay present irregularities, the water which 

 has percolated through the sandy bed will lodge in the 

 hollows, as at G. But when such cavities have become 

 filled, the water with which the rock is charged will flow over 

 them, and continue to run down in whatever direction the 

 rocks may chance to slope. 



It rarely happens that the successive layers of rock, or as 

 they are technically called strata,'^ exposed in any given 

 section are perfectly horizontal, or spread out with flat 

 surfaces, like the surface of a piece of still water. Generally 

 the beds slope or incline in a definite direction, and this 

 slope is technically termed the dip. If then we read in a 

 scientific description of a given section that " the strata dip 

 30° S.W." it means simply that the layers of rock slant in a 

 south-westerly direction, and make an angle of thirty 

 degrees with a perfectly horizontal surface. Thus, in the 

 diagram the dip is shown by the general direction of the 

 line CD, and its amount may be measured by the inclination 

 of this line to the horizon; that is, by the angle which the 

 line CD makes with the top or the bottom edge of the page, 

 when these edges are horizontal. Now when the water, 

 having percolated through the sandy rock A B C D, has 

 reached the junction represented in section by C D, it flows 

 down this plane in the direction of the dip, and escapes at 



' Stratum (plural strata) from the Latin stratum, signifj'ing that 

 which is extended or spread out. 



