224 é EDWARD B. MATHEWS 
Pikes Peak from the northwest at an elevation of 13,000 feet, 
which well illustrates this difference. On the west the mountain 
slopes with smooth rounded outline into the drainage of Beaver 
Creek, while on the east the descent is precipitous in ragged 
cliffs, sometimes resembling huge cyclopean masonry. Counter- 
acting this physical disintegration are chemical changes which 
Fic. 5.—Disintegrated bowlder of granite showing surface hardening and dis- 
integration beneath. 4 
protect the rock at first, but ultimately, in conjunction with the 
physical forces, accelerate the rate of rock-weathering. 
The effect of weathering extends for a distance of two or 
three feet beneath the surface of the exposed rocks. On the 
exterior there is frequently a dense crust, or glazing, rarely 
more than half an inch thick, covering a second zone several 
inches wide, in which the mineral are stained with iron and 
loosely held together. Beneath this zone the rock is often so 
incoherent that it seems ready to fall to pieces. The crumbling 
mass, in turn, passes gradually into the solid rock. Fig. 5 repre- 
sents a bowlder with the coating on the surface and the disinte- 
grating rock beneath. In this view the upper surface appears 
