WEATHEEIjSTG influenced by STRUCTUEE 229 



been broken up and washed awaj by currents of water, forma- 

 tions whicb are so bewildering, like the ridges (osars) and hills 

 with erratic blocks in Sweden and Finland, that I was aston- 

 ished when I saw them. ' ' 



The same features are brought out in the previous descrip- 

 tions relative to the weathering of the granite of the District 

 of Columbia, the diabase of Medford, Massachusetts, and other 

 localities mentioned in these pages. (See pp. 186 and 198.) 

 This tendency toward disintegration without decomposition is 

 exaggerated among coarsely crystalline rocks, as is abundantly 

 exemplified in the rocks of the Pikes Peak (Colorado) area. 

 Among those of finer grain, particularly the quartz-free varieties, 

 as the Fourche Mountain (Arkansas) syenites, decomposition 

 may follow so closely on disintegration that little or no sand 

 is formed, sound fresh rock passing within the space of a few 

 millimetres into the condition of residual clay.^ 



(2) Weathering influenced by Crystalline Structure.— it is 

 elsewhere observed that, other things being equal, a coarsely 

 granular rock will disintegrate more rapidly than one of finer 

 grain. 



Lone Mountain, one of the high eruptive peaks on the west 

 side of the Madison valley in Montana, presents in its upper 

 portions all the features of a volcanic crater broken down on 

 one side by the lava flow. The facts of the case are, however, 

 that the coarser grained central portion has been disintegrated, 

 and swept by wind and rain into the valleys, while the fine- 

 grained, more compact outer portions, those which solidified near 

 the line of contact with adjacent rocks, remain intact. Pro- 

 fessor BelP describes an interesting ease of this kind where the 

 coarsely crystalline central portion of a '^ greenstone" dike has 

 yielded more readily to erosion than at the sides and afforded 

 channel-way for the Mattagami River, north of Lake Huron, in 

 Canada. The gneiss adjoining the dike having been shattered, 

 yielded also to decomposing agencies and forms now a second 

 parallel channel on each side of the central one. ^^ Between them 



^ Br. Mas Fesca lias noted fhat tlie granitic roeks of Kai ptovinee, Japan, 

 yield on decomposing gravel, sand, and clayey loams, wMle those rocks poor 

 in quartz, sueh as the syenites, give rise only to clays (Abhandlungen nnd 

 Erlauternngen zur Agronomisclien Karte de a Prov. Kai, Kaiserlich Japan- 

 isehen Geologischen Eeichsanstalt, 1887). 



'Bull. Geol. Soc. of America, Vol. V, 1894, p. 364. 



