1918] 



Davis: The Franciscan Sandstone 



31 



products of decomposition by streams, the unweathered residuals of 

 feldspathic grains, tog-ether with quartz, might be segregated from the 

 products of decay. This concentration would give rise to a rock con- 

 taining fresh feldspar. This possibility is improbable in this instance 

 because of the marked angularity of the grains. This angularity is 

 opposed to any idea of origin which involves much sorting and 

 transportation. Also it is a well known fact that feldspar breaks 

 down very quickly under the abrasive forces acting in stream trans- 

 portation. 49 Such action would tend to produce a rock high in quartz 

 with only a small amount of feldspar. 



The presence of fresh grains of ferromagnesian minerals also 

 shows that the conditions under which the sandstone was formed were 

 not those of ordinary humid weathering. 



The mineralogical nature of the sandstone indicates, then, that it 

 was formed under conditions which favored the mechanical disintegra- 

 tion of the original rock and its separation into grains. After this 

 mechanical disintegration the constituent grains underwent little 

 chemical alteration during transportation and deposition. 



Barton 50 has investigated the special conditions under which 

 mechanical, granular disintegration of granitic rocks will occur. He 

 has summarized the localities where such granular disintegration 

 products are known, and investigated the climatic conditions under 

 which they were formed. 



His work shows that the granular disintegration of granitic rocks 

 may occur in a variety of climates. It is favored by high altitudes 

 where the daily temperature range is extreme. It appears that in 

 cold climates the rock breaks into large blocks ; there is little tendency 

 for granular disintegration, so that under such climates feldspathic 

 sand is not likely to occur. In humid, tropical climates, decomposi- 

 tion is so rapid that feldspathic sand will not accumulate. In humid 

 climates, therefore, accumulations of felspathic sand will be limited 

 to the temperate zone. In arid climates, granular disintegration may 

 occur either in the tropics or in the temperate zone. 



Barton finds that under aridity, all things combine to favor granu- 

 lar disintegration, together with the deposition of mineral grains in a 

 fresh condition, and under such a climate, arkose deposits may be very 

 extensive. 



49 Barrel, J., Climate and Terrestrial Deposits, Jour. Geol., vol. 16, pp. 364- 

 65, 1908. 



go Barton, D. C, The Geological Significance and Genetic Classification of 

 Arkose Deposits, Jour. Geol., vol. 24, p. 417, 1916. 



