274 M. I. Goldman — Catahoula Sandstone of Texas. 



moreover is the method employed by Thoulet and the one 

 followed in a study of the Upper Cretaceous sediments of 

 Maryland* so that direct comparison with the figures obtained 

 in the latter is possible. This comparison makes the abun- 

 dance of feldspars here even more striking, for the general ratio 

 in most of those Cretaceous sediments is 10 per cent feldspar, 

 running up to 25 per cent in only one sample. These Creta- 

 ceous sediments are undoubtedly all continental fringe deposits 

 (deltas, lagoons, estuaries, and the off-shore marine belt) so that 

 their bearing is significant. 



The ratio of 39 per cent in the Catahoula sandstone is 

 therefore seen to be high and may be taken to indicate, for 

 the present, merely that the sandstone was deposited near the 

 source from which its material was derived, and moreover 

 that the ratio is so high as to suggest a very f eld spathic rock 

 as that source. 



4. Weathering of the Feldspars. 



General appearance. — The brilliant freshness of many of 

 the feldspars even under the hand lens, where their lustrous 

 cleavage surfaces show up strikingly, seems very significant 

 aside from any attempt at a quantitative evaluation of the 

 degree of weathering. This alone may indicate origin of 

 the sandstone under conditions of mechanical rather than 

 chemical disintegration. Unfortunately, however, neither any 

 information I could obtain from the literature nor my own 

 experience enables me to assert that such feldspars can not 

 exist side by side with weathered ones in a sediment formed 

 under conditions of predominantly chemical disintegration. 



Ratio of fresh to weathered feldspars. — Quantitatively the 

 ratio of fresh to weathered feldspars is inconclusive. For 

 comparison there are available here the data gathered by 

 Mackie,f though his paper contains only two analyses of 

 sediments formed under conditions that may be regarded as 

 known. X One of these is a bowlder clay in which 86 per cent 

 of the feldspars are fresh, the other is a sample of sand 

 from the mouth of the river Spey in which 81 per cent of the 

 feldspars are kaolinized. The Catahoula sandstone lies half 

 way between them. With these analyses comes the general 

 statement that "Professor Judd found in the deposits of the 

 Nile delta a large proportion of remarkably fresh feldspars." 

 Here then are three distinct types of deposits represented ; 



* To appear in a volume on the Upper Cretaceous of Maryland, Maryland 

 Geol. Survey. 



f Mackie, Wm. : The feldspars present in sedimentary rocks as indicators 

 of the conditions of contemporaneous climate. Trans. Geol. Soc. Edin- 

 burgh, vii, pp. 443-468, 1898. 



tLoe. cit., p. 445. 



