GENESIS OF THE SOIL AND ITS POSSIBILITIES 179 



percolation of water is more easy as the displacement 

 approaches a vertical position. 



" A most remarkable example of this is seen in the 

 rocks of North Carolina. A kind of rock known as 

 trap is found in layers called dikes in the Newark 

 system of rocks in that State. These dikes have been 

 so completely displaced from the horizontal position 

 they at first occupied as to have an almost vertical dip. 

 The edges thus exposed vary from a few feet to nearly 

 one hundred feet in thickness. The trap rock in those 

 localities is composed almost exclusively of the mineral 

 dolerite, which is so hard and elastic in a fresh state 

 as to ring like a piece of metal when struck with a 

 hammer. In building a railroad through this region 

 these dikes were in some places uncovered to a depth 

 of forty feet and more. At this depth they were found 

 completely decomposed and with no indications of hav- 

 ing reached the lower limit of disintegration. The 

 original hard bluish dolerite has been transformed into 

 a yellowish clay-like mass that can be molded in the 

 fingers and cut like putty. Similar geological forma- 

 tions in New Jersey and further north do not exhibit 

 anything like so great a degree of decomposition, thus 

 illustrating in a marked degree the fact that freezing 

 weather for a part of the year is a protection against 

 rock decay. The ice of winter at least protects the 

 rocks from surface infiltration, although it cannot stop 

 the subterranean solution which must go on continu- 

 ously. 



" Other things being equal, therefore, it appears that 

 as the region of winter frost is passed the decay of the 

 rocks has been more rapid, because water, the chief 

 disintegrating force, acts more constantly. Decay of 

 rocks at the poles must be very slow. 



