proceedings: geological society 297 



on the fact that the sandstone has an opaUne cement which could be 

 decomposed by boiUng with KOH. 



The features observed and their interpretation were as follows: 



1. Size: Although there is a marked predominance of 0.5 to 0.25 mm. 

 grains, it is not as pronounced as in tyjDical beach sand, resembling 

 rather wind bloA\ai sand such as seems to be produced b}- the strong winds 

 of the tropics. 



2. Degree of rounding: Some grains show wonderfully perfect round- 

 ing, thus suggesting wind action. 



3. Lower limit of rounding: This is the essential factor in rounding, 

 since it is in this that the action of wind and water are believed to differ. 

 The smallest well rounded grains found measure 0.035 mm. diam. 

 This corresponds to the lowest limit found bj' Fruh in some Pampas 

 soil and is therefore believed to indicate aeolian origin, though the 

 lowest limit for water rounding is not knctvai. 



4. Proportion of rounded grains: This is low (28.7 per cent) and in 

 view of the perfect rounding of some of the grains is taken to indicate that 

 fresh angular material was constantly added, which would correspond 

 to the conditions in an arid region. 



5. Weathering of the feldspars: Many of these seem remarkably 

 fresh, suggesting the mechanical disintegration of an arid region. 



6. Proportion of weathered feldspars: About half are weathered, 

 which, according to Machie's studies, is too much for an arid region, 

 but this condition may have been produced after deposition. 



7. Proportion of different species of feldspar: Orthoclase 85 per 

 cent, microcline 13 per cent, plagioclase 2 per cent. The low plagio- 

 clase suggests conditions of humid weathering which as stated under 

 (6) may have been subsequent to deposition. 



8. Ratio of feldspar to quartz: Feldspar forms 39 per cent of the 

 mass, which is a high percentage for sandstones and like (4) suggests 

 constant addition of fresh material as under arid conditions. It also 

 is evidence against the production of the sizing by beach waves and in 

 favor of wind. 



9. Proportion of heavy minerals: In the portion between .05 mm. 

 and 0.25 mm. diameter the heavy minerals amount to 1.1 per cent. 

 This is ver}^ low for aqueous deposition, high for a mature desert dune, 

 but might correspond to an arid region where the supply of fresh material 

 is kept up. 



10. Species of heavy minerals: The dominant heavy minerals are 

 magnetite and zircon. Zircon is a mineral very resistant to alteration, 

 agencies, and magnetite may also be. Practical absence of epidote 

 and chlorite indicates arid conditions. 



11. Ratios of heavy minerals to each other: The insignificant quan- 

 tity of fissile minerals like the micas and ferromagnesian minerals 

 strongly indicates arid conditions. 



12. Bedding: Absence of thin bedding indicates rapid accumulation 

 by wind or water currents. 



13. Arrangement of fossils: (a) Curled leaves and prints arranged 

 at random point to burial by wind, (b) bunches of leaves fairly flat and 



