248 THE PHYSICAL MANIFESTATIONS OF WEATHEEINa 



In regions of gneissic or granitoid rocks traversed by large 

 veins of quartz, as in the northwestern part of the District of 

 Colnmhia, the superior resisting power of the quartz causes it 

 to stand out in relief from the gradually dwindling rock masses 

 on either hand, giving rise thus to prominent knolls, or ridges, 

 the occasion for which is a mystery until we come to examine 

 the foundation materials. Belt, in describing the auriferous 

 quartz lodes at San Domingo,^ states that the prevailing trend 

 of the main ranges is nearly east and west, and is probably due 

 to the direction of the outcrops of the lodes which have resisted 

 the action of the elements better than the soft dolerites. 



So striking a feature of the landscape as the Devil's Tower 

 or Bear Lodge on Little Sun Dance River, Wyoming, is due to 

 the weathering away and erosion of sedimentary beds from 

 around a dense crystalline core or plug of eruptive rock in- 

 truded into them in some past period of volcanic activity. 

 Through its greater powers of resistance, this still stands, 

 towering over 1000 feet above the level of the river, though in 

 time this, too, must go. Quite similar forms have resulted, 

 within a comparatively brief geological period through the 

 erosion of tufaceous cones from around the compact, crystalline 

 plugs of lava which solidified within the crater when volcanic 

 activity ceased. Beautiful examples of these are to be seen in 

 Arizona and New Mexico, where they are known as * Volcanic 

 necks." The formation of bosses through the influence of 

 joint planes has been described elsewhere (p. 231). 



In regions abounding in intrusive olivine or pyroxene rocks 

 which have undergone alteration into serpentine and talc or 

 '^soapstone,'' one frequently finds these materials forming the 

 main mass of the hills, while the valleys are carved out of the 

 softer, more readily decomposed granite, or whatever the country 

 rocks may be. The same feature is prominently developed in 

 the slate regions of Harford County, Maryland, where the slate 

 is the more eiiduring rock, and forms steep ridges, flanked by 

 valleys, carved out from less resisting materials. Regions of 

 trappean dikes in siliceous schists or gneisses, particularly 

 along sea-shores where swept by incoming tides, are often 

 characterized by narrow, straight-walled chasms, or canons due 

 to the weathering out of the basic rocks, while the more refrac- 

 tory schists on either hand remain. 



^The Naturalist in Nicaragua. 



