PRIXCIPLES OF ROCK WEATHERING 865 



Indeed it is ordinarily assumed that the silicate of alumina is not 

 at all affected, but the researches of Miiller, to which we have 

 referred, seem to disprove this, as do also the calculations made 

 in j)revious pages. Those silicates which are least liable to atmos- 

 pheric decomposition are, as it is to be expected, those which 

 have resulted from the alteration of less stable silicates, as ser- 

 pentine from olivine, epidote from hornblende, or kaolin from 

 feldspar, etc. So much is this so that serpentine has been called 

 a final product of alteration. A few silicates like tourmaline and 

 zircon, or garnet, or oxides like rutile and magnetite, or the salts 

 of rarer earths like monazite, etc., are scarcely at all affected 

 by any of the ordinary products of decomposition, but remain in 

 the form of residual sands in the beds of streams, from whence 

 the lighter, more decomposed, material is removed by erosion. 



In the weathering of potash feldspar rocks carr3dng black 

 mica, the latter mineral is, as a rule, the first to give way, and at 

 times almost wholly disappears. With basic rocks, on the other 

 hand, the dark mica is one of the most enduring of the constitu- 

 ents, and in the residual sands may be found in surprisingly large 

 proportions. 



Among the feldspars the potash varieties are, as a rule, far 

 more refractory than the soda lime, or plagioclase, varieties. 

 This is shown not merely by our own investigations, but by 

 those of others as well. Roth shows ^ from analyses of fresh 

 and weathered phonolites, nepheline basalts, and dolorites that 

 the loss of soda is almost invariably greater than that of potash. 



Indeed as shown in our analyses the potash feldspars may 

 lose very little by decomposition, but be converted into the con- 

 dition of fine silt merely through a mechanical splitting up. 

 This fact will in part explain the relative scarcity of free potas- 

 sium salts (carbonates, sulphates, and nitrates) as compared with 

 those of soda.^ 



'Op. cit., 3d ed., 2d Heft. 



^An oligoclase occurring in a tourmaline granite on the southern slope of Mt. 

 Mulatto, near Predazzo, undergoes, according to Lemberg (Zeit. derdeutsch. geol. Ge- 

 sellschaft, 28, 1876), a much more rapid decomposition than the orthoclase with which 

 it is associated, and gives rise to a green lusterless serpentine-like product. The 



