162 Study of the New York Obelisk as a Decayed Boulder. 



must allow for differences in constitution of the rock, in considering 

 such small quantities, and for unknown variations in the length of 

 exposure of these specimens to the weather. 



(1). This granite, from whatever source derived, is bv no means 

 a compact mass, but is traversed by interstices in notable propor- 

 tion, amounting to (see column e on total voids) from one-half to 

 over one per cent, of its volume, according to its fresh character or 

 condition of incipient decay. 



In other words, even the dense Syene granite is finely spongy 

 throughout, in its freshest state. 



(2). In regard to the rock aggregate, the fresh granite (c) from 

 the Obelisk, probably broken from the ever sheltered heel of its 

 shaft, apparently represents either accidentally the most compact 

 variety, or else the freshest condition of the Syene granite in my 

 series, retaining the lowest coefficient of lateral absorption (.083), 

 i. e., the smallest proportion of rock-pores, about I of one per cent, 

 of the volume (.216). But in regard to the constituent minerals, 

 the fresh granite, as just arrived from Alexandria, contained nearly 

 50 per cent, more voids (716 to 517) than that at Syene, chiefly in 

 its more abundant mineral interstices. This may indicate the effici- 

 ency of hydration in the damper climate of the Eg3"ptian sea-coast. 



(3). The progress of decay of the surface chips of the granite, in 

 the quarry at Syene, was attended with increase in the minute inter- 

 stices of its component minerals rather than in the pores of the rock ; 

 the original relationship (columns g and /) 16 to 84 became 28 to 72. 



This seems to show that, in the arid climate of Syene, the chief 

 element of decay in the granite was chemical, consisting in the ab- 

 sorption of oxygen and water by its minerals. The limited absorp- 

 tion of the latter, however, is shown by the determination of the 

 loss by incineration at 0.65 per cent., and in the microclin at 0.35 

 per cent.' 



(4). The progress of decay in the granite of the Obelisk, on the 

 other hand, from 1881 to 1885, has yielded an increased proportion 

 of rock-pores ; the relationship of 70 to 30 having changed to 63 to 

 37. The mineral voids have increased 50 per cent. (.500 to .759), 

 and the rock-voids have more than doubled (.216 to .461). 



This indicates the action of a chemical force on the minerals, 

 increasing their clefts, and a still more efficient mechanical action ; 



1 Delesse, loc. cit., 489. 



