332 G. p. MERRILL — DISINTEGRATION OP GRANITIC ROCKS. 



find reference to like phenomena in existing literature I may be excused 

 for describing it somewhat in detail. 



While excavating in the tunnel for the water-works extension in Wash- 

 ington, sharply angular natural joint-blocks of granitic and dioritic rocks 

 with smooth, even faces, were brought to the surface from varying depths 

 up to a hundred and some odd feet. Much of the material was perfectly 

 fresh and sound, and has been utilized for road-making and building 

 purposes. Much, on the other hand, while apparently fresh and show- 

 ing on casual inspection no signs of decomposition, gave forth only a dull 

 sound when struck with a hammer and showed a lusterless fracture. 

 Blocks of this last type nearly always rapidly disintegrated into coarse 

 sand after short exposure, though manifesting no other sign of mineral- 

 ogic change than a whitening of the feldspars. So marked was this feat- 

 ure that even the workmen noticed it, and on more than one occasion 

 samples of this or the sound rock were brought me by builders who 

 questioned its durability, inasmuch as some of the material " slacked 

 like lime," as they expressed it, on exposure. 



My explanation has always been that the various minerals composing 

 the rock (with the exception of the quartz) underwent a partial hydra- 

 tion from percolating waters, but, held in the vise-like grip of the sur- 

 rounding rocks, were unable to expand to the extent of loss of cohesion 

 and consequent disintegration. As soon as freed from compression ex- 

 pansion and presumably further hydration took place, the mass became 

 spongy, and, freely absorbing water, fell into sand and gravel. 



This idea led me to make a few experiments toward ascertaining the 

 actual amount of expansion the rock undergoes during this transforma- 

 tion. Barring the error due to loss of material by solution, it is evident 

 that a fair approximation may be gained by a comparison of the weight, 

 bulk for bulk, of the fresh and decomposed material. It being obvious 

 that in order to fulfill existing conditions no great refinement of methods 

 was essential, I contented myself with taking a quantity of the air-dried 

 material and measuring it in straight, cylindrical glass vessels, bringing 

 it to the approximate condition of the soil b}^ tamping with water, and 

 afterward drying and weighing. By comparing the weights per cubic 

 centimeter thus obtained with the weight of a cubic centimeter of the 

 fresh rock, as shown by its specific gravity, I was able from an aver- 

 age of several determinations to obtain an approximation of 1.88, which 

 represents with a fair degree of accuracy the average amount of expansion 

 which the rock has here undergone in passing from its fresh condition 

 into that of undisturbed soil a foot beneath the surface. 



