238 THE PHYSICAL MANIFESTATIONS OF WEATHEEINa 



binding eonstituents whereby all cohesion is lost, and disinte- 

 gration follows from necessity. (Fig. 1, PL 22.) 



This form of dismtegration seems to take place only in boulders 

 exposed at or near the surface, and is believed to be due pri- 

 marily to expansion and contraction from alternations of tem- 

 perature. 



Many roek^, owing to a lack of homogeneity, weather with 

 extreme irregularity and give rise to odd and sometimes fan- 

 tastic forms. In the case of a friable sand or limestone, sub- 

 ject to wind or rain erosion or to solution, certain portions may 

 be protected by a capping of other rock while the intervening 

 material is carried away. There thus arise spindle-shaped 

 forms of varying proportions, each capped by the roof or hat- 

 like block to which it owes its origin. Such have been noted 

 in many regions, and have been described by Hayden as occur- 

 ring on a colossal scale in Colorado. 



(5) Results due to Position. — In very many instances loose 

 blocks of stone lying exposed upon the ground will undergo 

 a more rapid disintegration from the lower surface, a feature 

 evidently due to the fact that this portion of the rock is kept 

 in a state of continual moisture. This form of disintegration 

 results in the production of oval, flattened, scale-like masses, 

 quite independent of the original jointing. In other cases 

 decomposition going on from all exposed sides of a joint block 

 may in time produce the so-called rocking-stones or '^logans" 

 and ^Hors" of English writers, though some of these are un- 

 doubtedly nicely balanced boulders from the glacial drift. 



A mass of rock may be prevented from undergoing disinte- 

 gration, even though partially decomposed, by its surroundings. 

 Thus, in driving the tunnel for the waterworks extension, in 

 "Washington, natural joint blocks of hard and apparently firm 

 rock brought to the surface would fall away to loose sand in 

 course of a few days, or months, as the case might be, much 

 depending on the conditions of the weather and the state of 

 decay. This characteristic was sufficiently pronounced to attract 

 even the attention of the workmen, who described the rock as 

 ^' slaking" and believed it to contain quicklime. 



The fact was that percolating waters had brought about a 

 partial kaolinization of the feldspar, and hydration, without 

 great oxidation of the iron-magnesian constituent. The original 

 pressure, coupled with that incidental to expansion from hydra- 



