Ch. 13] MINERAL CONTENT 233 



tion and consolidation, and thus affects the density and shearing 

 strength of the material. Laboratory tests show that, for sands of 

 high roundness and sphericity, the density and porosity are quite 

 sensitive to the rate of deposition (Kolbuszewski, 1948), slow deposi- 

 tion producing the denser deposits. The shape of mineral particles 

 may also affect stability through preferred orientation. The writer 

 has seen numerous rockslides along bedding planes in dipping sand- 

 stone near Glenwood Springs, Colorado. The bedding planes along 

 which slippage occurred were covered with mica flakes lying in the 

 plane of the bedding. 



Mineral Content 



Glauconite, a fairly common mineral in sedimentary rocks, may 

 contribute to landsliding. An interesting example of a slide due to 

 glauconite in the cliffs around Algiers Bay was described by Proix- 

 Noe (1946) and by Drouhin, Gautier, and Dervieux (1948). The 

 cliffs around the bay are composed of glauconitic marl underlying 

 sandy limestone. Water percolating down through the limestone 

 became charged with calcium carbonate. As the water entered the 

 marl and contacted the glauconite, the calcium ions were fixed and 

 potassium ions liberated The water became markedly more alkaline 

 (pH 9), resulting in the deflocculation of the marl and the hydrolyza- 

 tion of alumino-silicates. The marl was made highly fluid and could 

 no longer support the overlying beds, so the cliffs collapsed. Glau- 

 conitic mudstones and greensands have been involved in landslides 

 in New Zealand (Benson, 1946) and along the seacoast of England 

 near Folkestone (Toms, 1948) although no reference was made in 

 either description to possible physicochemical action of the glauconite 

 itself. 



Gypsum also may contribute to landsliding. It is moderately 

 soluble, and its removal by circulating waters within a series of sedi- 

 mentary strata may cause subsidence, or sliding if a steep face is ex- 

 posed. It also has two other effects of quite different natures. The 

 dissolving of gypsum disseminated through a shale, and its subse- 

 quent recrystallization along minute fractures close to the ground 

 surface, are potent factors in breaking up and rendering permeable an 

 otherwise rather massive and impermeable clay or shale. This process 

 has destroyed effectively a grout surfacing placed on Mancos shale 

 slopes at Mesa Verde National Park to prevent the shales from being- 

 wetted by rain. At that place, water could get at the shale behind 

 the roadcut from an overlying permeable and fractured sandstone 

 (Varnes, 1949). Even in flat ground, the crystallization of gypsum 



