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POPULAR SCIENCE MONTHLY. 



posed is like that at the bottom of the quarry. As we ascend from 

 the point indicated by the lower hammer, we notice that the dark- 

 blue rock gradually takes on a rusty hue, and its toughness has be- 

 come less. Going still higher, the rusty character increases, and 

 along joints the rock is so lacking in coherency as to fall to pieces 

 when struck a light blow with a hammer. The central portions of 

 the blocks, however, after we have removed the outer shell of rusty 



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Fig. 2. — Detailed View of a Portion of Quarry showing Weathered Kock. 



material, are seen to be like the lower rock. In the middle fore- 

 ground of the picture there are shown several bowlders derived from 

 above, which are merely these residual cores, and are known as 

 bowlders of disintegration. These are also shown in place near the 

 top of the picture at the extreme left. Xear the top of the quarry, at 

 a point marked by the upper hammer, the solid rock gives place to a 

 rusty mass of loose material, traversing which the cracks may still be 

 seen, and in which there are few indications of the solid rock * (see 

 Fig. 2). This loose material when carefully examined is found to 



* The position of the solid rock is shown by the hammer at the extreme right, standing 

 vertically. 



