WEATHERINa INFLUENCED BY COMPOSITION 237 



In any rock consisting of a variety of minerals, disintegration 

 is likely to constitute a more prominent feature of weathering 

 than in one of less complexity of composition, owing to the 

 unequally refractory properties of its constituents. Thus a 

 granite must yield a sand, while a purely feldspathic, pyrox- 

 enic, or calcareous rock may yield only clays. 



Beds of feldspathic quartzite, through the decomposition of 

 the feldspar, undergo disintegration, giving rise to beds of 

 friable siliceous sand interlaminated with kaolin, as described by 

 Dana.^ The same author also describes an interesting pseudo- 

 breccia formed by a quartzite divided up by a succession of 

 cracks into which limonite from decomposing pyrite has fil- 

 tered and acted as a colored cement. He says: '^Many of the 

 pieces lie in place barely separated from one another, and ap- 

 pear to be undergoing new divisions. But in the lower part, 

 large pieces look as if there had been wide displacements; yet 

 the hardly disturbed condition of the upper half proves that 

 the apparent displacement is due to the extension of the color- 

 ing and penetrating limonite. The cracks are made in part 

 by the extremely slow, wedge-like action of the depositing 



Ileusser and Claraz^ describe somewhat similar breccias 

 formed in Brazil through the weathering of crystalline schists 

 rich in iron. These breccias consist of angular fragments of 

 schist, more or less decomposed, firmly cemented by limonite. 



The boulders of Oriskany quartzite in the Cretaceous gravel 

 about Washington, District of Columbia, are composed of 

 rounded and angular quartz fragments tightly bound together 

 by a fine granular crystalline aggregate of quartz and feldspar. 

 Disintegration first manifests itself on the exterior of the 

 boulders in the form of an irregular network of grooves or 

 channels, which gradually become more and more conspicuous 

 until the boulder falls into bluntly pyramidal fragments and 

 finally into sand. The microscope shows that the disintegra- 

 tion is due to the disaggregation and partial kaolinization of the 



tion of their origin offered. Similar cavities have been also described in 

 Madagascar by the Eev. Barron who ascribes their formation to the pres- 

 ence of imprisoned vapors in the original rock magma; in other words, 

 to be comparable to the vesicular cavities in lavas. 



^Am. Jour, of Science, Yol. XXVITI, 1884. 



=*Ann. des Mines, 5th Series, Yol. XYII, 1860, p. 290. 



