Page:Popular Science Monthly Volume 54.djvu/478

460 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 become 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



material, are seen to be like the lower rock. In the middle foreground 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. Near 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