THE WEATHERING!- OF ROOKS (Continued) 



III. THE PHYSICAL MANIFESTATIONS 



Eock-weathering manifests itself in a great variety of ways, 

 much, depending upon climate, though naturally the controlling 

 factor is that of mineral composition. The manner of weather- 

 ing is often suiSciently characteristic to be of great importance 

 in determining surface contours, as well as incidentally afford- 

 ing a means for the identification of rock masses when the 

 outcrops themselves are obscured by decomposition products. 

 Such a means is of only local importance, however, since under 

 varying conditions the resultant forms assumed, evk by similar 

 rocks, are themselves quite variable. It is, nevertheless, not 

 without interest to note the ^'arying phases of weathering in 

 different kinds of rocks, the incidental contours assumed, the 

 character of the resultant debris, and, at the same time, the 

 controlling forces that have been instrumental in bringing about 

 the final result. 



(1) Disintegration without Decomposition. — That in weath- 

 ering, physical and chemical agencies may go on either singly 

 or conjointly has been noted in previous pages. In the case of 

 single minerals, the preliminary disintegration is beautifully 

 illustrated in the large oligoelase masses associated with micro- 

 cline in the feldspar milties of Delaware County, Pennsylvania. 

 In the dumps of waste about the mines these are found, in all 

 stages of disintegration, the mineral splitting up along cleavage 

 lines, becoming slow-white, and ultimately falling away to a 

 kaolin-like product, but which, when submitted to microscopic 

 examination, is found to be made up of sharply angular cleavage 

 particles, showing little sign of decomposition other than that 

 indicated by occasional opacity. In the analyses given on the 

 next page are shown (I) the composition of a fresh oligoelase 

 (as given by Dana) from near "Wilmington, Delaware, (II) the 

 snow-white cleaved, but still moderately firm mineral mentioned 

 above, and (III) the flour-like or kaolin-like product. 



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