THE KOCKS OF THE WASIIOK DISTRICT. (j5 



of decomposition is very different, from that wliicli it undergoes when l)uried 

 beneath the surface. Croppinrrs of tlie fresh rock rarely exliibit the tendency 

 to divide into parallel jjlates so characteristic of the other trndesite. The want 

 of homogeneity in structure displays itself in a different but very interesting 

 manner. Under the action of the weather it frequently becomes apparent 

 that large masses of augite-andesite are composed of thin beds of various 

 character. Some of these yield to weathering much more rapidly than 

 others, and the exposed face becomes indented with closely set parallel 

 grooves, such as are often observed in finely laminated sedimentary rocks. 

 There is, however, no perceptible tendency to the development of cracks in 

 the directions indicated by these grooves. The most natural explanation of 

 this structure would seem to be that they represent rapidly succeeding flows 

 of the melted rock, but it is hard to see in that case why differences of ten- 

 sion do not lead to the development of fissures. Other masses show an 

 analogous but different behavior in the development of grooves of sinuous 

 form, which cross each other at considerable angles, and give the surface 

 somewhat the appearance of an irregular pavement. If this structure were 

 found only upon opposite surfaces of blocks, it might be interpreted as an 

 expression of a tendency to separate into columns ; but when it occurs at 

 all, it is found equally on all the faces exposed. It appears to me that 

 solidification must have set in from numerous centers distributed through the 

 rock, giving it a coarse pseudo-spherolitic structure, and that the grooves 

 must repi'esent a slight difference in chemical composition in that portion of 

 the lava which was the last to solidify. Whatever may be the cause of the 

 appearance, it is highly characteristic of the rock in this District. A good 

 example appears in the foreground of the frontispiece. 



When fresh augite-andesite is exposed to the air, it soon becomes coated 

 with a yellowish-white product of decomposition. This is gradually con- 

 verted uito a bright reddish-brown substance, no doubt largely ferric oxide, 

 the surface at the same time growing rough. In many cases this color is 

 succeeded later by a pitchy black. The rate of change is by no means 

 slow, and in some of the railroad cuts, made a dozen years since, decom- 

 position has penetrated the I'ock for about a quarter of an inch. There is 



reason to suppose that after the rock has turned black the rate of change 

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