CHEMICAL ACTION 01^ WATEE 167 



opinion, of more importance than is ordinarily supposed* Gran- 

 itic rocks in the District of Columbia have been shown^ to have 

 become disintegrated for a depth of many feet with loss of but 

 comparatively small quantities of their chemical constituents 

 and with apparently but little change in their form for combina- 

 tion. Aside from its state of disintegration, the newly formed 

 soil differs from the massive rock, mainly in that a part of its 

 feldspathic and other silicate constituents have undergone a 

 certain amount of hydration. Natural joint blocks of the rock 

 brought up from shafts were, on casual inspection, sound and 

 fresh. It was noted, however, that on exposure to the atmos- 

 phere such shortly fell away to the condition of sand. Closer 

 inspection revealed the fact that the blocks when brought to the 

 surface were in a hydrated condition, giving forth only a dull, 

 instead of clear, ringing sound, when struck with a hammer, and 

 showing a lustreless fracture, though otherwise unchanged. 

 That such had not previously fallen away to the condition of sand 

 was evidently due to the vice-like grasp of the surrounding rock 

 masses. These observations seem to have since received confir- 

 mation from Professor Derby ,^ who states that the sedimentary 

 rocks of Sao Paulo, Brazil, as seen in the deep railway cuttings, 

 ''are almost invariably soft even when they show no signs of 

 decay, and go to pieces by a kind of slaking process when 

 broken up and exposed to the air, though they may have 

 required blasting in the original opening of the cuttings." 

 Professor W. 0. Crosby*^ gives it as his opinion that the dis- 

 integration of the Pike's Peak (Colorado) granite is due mainly 

 to hydration, the mica particularly being affected. 



Professor Alexander Johnstone showed* by experimentation 

 that normal muscovites, when submitted to the action of pure 

 and carbonated waters for the space of a year, underwent very 

 little change other than hydration, and a diminution in lustre, 

 hardness, and elasticity. They appeared, in fact, to be converted 

 merely into hydromuseovites, the hydration in pure water hav- 

 ing gone on nearly as rapidly as in that which was carbonated. 

 Biotite, when similarly treated, showed a slight discoloration 

 or bleaching on the edges, accompanied also by hydration, and, 



1 Bull. Geol. Soc. of America, Yol. YI, p. 321. 



^ Deeomposition of Roeks in Brazil, Jour, of GeoL, YoL lY, 1896, p. 205. 



3 Personal Memoranda to the Writer. 



^ Quar. Jour. Geol. Soc. of London, Yol. XLY, 1889. 



