WORK OF THE ATMOSPHERE 33 



regions such as Pike's Peak, the granite countries along the Colo- 

 rado River in Arizona, the peaks of Mount Sinai, and many other 

 mountain tops where taluses or screes of crystalline sand, made up 

 of dissociated minerals of the granite, characterize the exposed 

 movuitain sides and fill up all the hollows. The feldspar of this 

 sand is generally fresh and unclouded, the original crystals being 

 scarcely altered. Such sands, reconsolidated with little assortment, 

 give rise to peculiar arkoses in which the fresh feldspar crystals 

 form the most striking feature, and may even cause the rock to be at 

 first mistaken for an unaltered granite. Examples of this kind are 

 found in certain parts of the Torridon sandstone (pre-Cambric) of 

 the west of Scotland, and in other rocks. Granite bosses, wherever 

 exposed, are generally marked by rounded surfaces, intersected per- 

 haps by joint cracks into which the disintegration products have 

 been washed, and which alone support vegetation, the granite sur- 

 face being bare. Granite boulders exposed to the same forces will 

 crumble into heaps of crystalline sand. Since the corners and 

 angles of rock fragments are their most exposed portions, the first 

 being attacked from three sides, it follows that disintegrating blocks 

 will soon assume a rounded outline. Concentric exfoliation of the 

 disintegrated outer portions of such rock masses is observable, the 

 rock becoming a boulder formed in situ and often embedded in a 

 mass of disintegrated and partly decomposed residual sand and soil. 

 Branner, discussing the effect of these temperature changes on the 

 rocks of Brazil, remarks : "The unec^ual contraction and ex- 

 pansion of the minerals composing the rock tend to disintegrate the 

 entire mass, while the even annual and diurnal changes and the 

 approximately even penetration of these changes cause the rock 

 to exfoliate or to spall off in layers of even thickness, like the coats 

 of an onion, while the crevices opened in the rocks admit acids and 

 gases and set up a train of reactions that sooner or later disinte- 

 grate and decompose the entire rock mass." (10:281.) 



The individual minerals themselves may likewise be affected by 

 this change in temperature, since there is differential expansion 

 along the dift'erent crystal axes. A cleavable mineral like feldspar 

 may thus become potentially shattered, innumerable fine rifts form- 

 ing along the cleavage planes, which, though insufficient to cause 

 the mineral to crumble, yet admit air and moisture and permit 

 chemical decomposition within the crystal. In moist climates this is 

 seen in the clouding of a feldspar crystal along the fine cleavage 

 lines as seen in a thin section under the microscope. 



Disintegration, as here described, is always associated with a 

 greater or less amount of chemical alteration, such as oxidation, 



