LOCAL DECOMPOSITION OF ROCK BY THE CORROSIVE 

 ACTION OF PRE-GLACIAL PEAT-BOGS 



EDWIN W. HUMPHREYS AND ALEXIS A. JULIEN 



While the layer of decayed rock which once overlay the region 

 around New York City has been generally planed off by the conti- 

 nental glacier, certain small isolated spots have been noted from 

 time to time in which masses of rotten schist still remain. Their 

 decay is commonly attributed to weathering action, and their escape 

 from the glacial scour at such points, to their probable protection 

 by projecting eminences of rock under whose lee they are supposed 

 to lie. 



Excavation in schist. — An unusually large occurrence of this 

 kind has been recently exposed in an excavation for a cellar on the 

 east side of the junction of Southern Boulevard and Westchester 

 Avenue, Borough of the Bronx, New York City, whose general 

 form and dimensions 1 are shown in Fig. i. 



The gneissic schists here present the foliation with usual high 

 angle, 70 to 90 ; strike N. 23 E. and S. 23 W. A small anticlinal 

 fold crosses the strata, as shown in the diagram (Fig. 1) whose 

 axis runs N. 52 E. and S. 52 W. A small overthrow is shown in 

 its cross-section at the northern end, and at its southern end it 

 pitches to the southwest at an angle of about 30 . The rock con- 

 sists chiefly of a fine granular aggregate of quartz, with much 

 biotite in minute black scales, and more or less disseminated white 

 feldspar. Throughout the western half of the excavation, however, 

 many thin seams of pegmatitic gneiss and of gray quartz are 

 intercalated, up to nine inches in thickness. 



Pegmatite dike. — A pegmatite dike, about five feet in width, 

 nearly vertical, also cuts obliquely through the schists, with a course 

 of N. 30 E. and S. 30 W. At many points, small projections or 

 apophyses branch out into the schist along its course and are, 



1 We wish to express our indebtedness for these data to Mr. C. S. Shumway, 

 superintendent of the Construction Department of the American Real Estate Co. 



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