DECOMPOSITION OF ROCK BY CORROSIVE ACTION 



51 



mark the cross-section by a series of conspicuous, white, kaolinic, 

 lenticular bands, contrasting with the intervening layers of dark 

 till. 



The bowlder clay 

 of these intervening 

 layers is very dense 

 and compact, some- 

 times sandy, some- 

 times rich in mica 

 and clay, and con- 

 tains few pebbles 

 and occasional bowl- 

 ders up to about two 

 feet in diameter, 

 which may show 

 sharp, glacial striae. Fig 3 _ Manner in which the pe gmatite dike was 



These Consist partly planed off by the glacier. 



of rocks of the vicin- 

 ity, quartz from seams, granitoid and hornblende gneiss, etc., and 

 partly of rocks from the Palisades on the west bank of the Hudson 



River, about five 

 miles distant, viz., 

 diabase, coarse red 

 sandstone, indurated 

 shale from the con- 

 tact underlying the 

 trap, etc. Nearly 

 all these bowlders 

 are hard and un- 

 decomposed. 



At several points 

 where such a trap 

 bowlder rested im- 

 mediately upon a 

 granite slab, the 

 latter was deeply indented, its folia separating and rising a little 

 around the bowlder, above the upper level of the slab. Immediately 



Fig. 4. — Section along Westchester Avenue. 



