236 THE PHYSICAL MANIFESTATIONS OF WBATHEBING- 



the individual granules, as may be observed in many an old 

 tombstone or polished column. 



Even where the decomposition is almost purely chemical, the 

 corroded surfaces are peculiarly irregular, as shown in PL 14. 

 This feature is doubtless due to some imperceptible difference in 

 the texture of the stone, or to the presence of joints and flaws 

 which give direction to the solvent fluids. Prof. C. H. Smyth 

 has called the author's attention to the fact that in crystalline 

 limestones weathering commonly proceeds most rapidly along 

 cleavage lines and the -JR twinning planes. Calcareous rocks 

 consisting of an admixture of ealcite and dolomite crystals may 

 undergo disintegration through a complete or partial removal of 

 the ealcite granules by solution, the dolomite remaining almost 

 untouched- Certain dolomitic limestones near Stockton, Min- 

 nesota, have been described^ as peculiarly subject to this form 

 of disintegration. The mass of the rock consists of dolomitic 

 crystals and granules, but often interlaminated with narrow 

 bands of caleite. Through the removal of the latter,- the 

 stone becomes porous and its degeneration so complete that 

 '^shovelfuls of loose sand consisting of dolomitic rhombohedra 

 can be taken up. ' ' 



Fine-grained, compact, and seemingly homogeneous rocks 

 may, on account of imperceptible differences in composition 

 and structure, weather out in strikingly irregular and peculiar 

 forms. Compact limestones and other rocks losing materials 

 chiefly by solution sometimes give rise to markings so closely 

 resembling hieroglyphic or cuneiform characters, that it is not 

 surprising they have more than once been mistaken for the work 

 of human hands. 



Massive granitic rocks seemingly of quite uniform composi- 

 tion will sometimes weather very irregularly, giving rise to 

 oven-like cavities, in general shape resembling the pot-holes in 

 the beds of streams. Eeusch has described^ such in exposed 

 faces of granite ledges on the island of Corsica, the holes 

 extending inward horizontally, or sometimes with an upward 

 tendency. The cause of this is not apparent from the descrip- 

 tion given, but it is presumably due to slight textural differences 

 such as are not readily discernible in the decomposed rock.^ 



^ Hall and Sardeson, Bull. Geol. Soc. of America, Vol. YI, 1895, p. 184. 



^ Forhandlinger i Videnskabs-Selskabet i Christiania, 1878, No. 7, pp. 

 24-27. 



* These cavities tave since been described and figured by F. F. Tuckett 

 and T. G. Bonney (Geol. Mag., Vol. I, 1904) but* no satisfactory explana- 



