DECOMPOSITION OF ROCK BY CORROSIVE ACTION 



49 



in the cross-section at the north end (at the point B), the hand 

 placed across this line would rest on the left upon the decomposed 

 schist, easily dislodged by the touch of a finger, and on the right 

 upon the hard fresh rock. This contact is shown in Fig. 2, with 

 the decayed schist on the left, and on the right the same rock in 

 rugged, hard condition. The trend of this sharp division line was 

 N. 28 E. and S. 28 W., approxi- 

 mately parallel to the course of 

 the dike. However, in the cross- 

 section, a few seams of decayed 

 rock were noticed to the east of 

 this line, descending a yard or 

 more into the solid schist. The 

 same section showed that the 

 upper eroded surface of the schist 

 descended from a height of four- 

 teen feet at the point B along 

 the northeast wall, to a height 

 of seven and one-half feet, in a 

 distance of fifty-four feet to the 

 corner on Westchester Avenue. 



Decay of pegmatite. — A similar 

 decay has affected the pegmatite, 

 much of whose feldspar has passed 

 into a white kaolinic clay, so 



that this rock also was easily removed by means of the pick. 

 Although it is even now much more tough and solid than the sur- 

 rounding schist, it appears to have been planed off by the ice at 

 about the same level, as shown near the bottom of the cross-section 

 (Fig. 3) where the north end of the dike strikes the wall at West- 

 chester Avenue. Above it lies a layer of till, and then a slab of 

 granitic gneiss. It should be also noted that the decay above 

 described is entirely exceptional in this region. For example, in 

 another excavation in the schist, a few hundred feet to the north, 

 the same schist was found practically undecomposed and sound. 

 So also as to the numerous other pegmatite dikes in the Bronx, all 

 we have observed are solid and show almost no decay. 



Fig. 2. — Contact of decomposed and 

 undecomposed schist. 



