M. I. Goldman — Catahoula Sandstone of Texas. 265 



Ratio of Feldspar to Quartz. — A count in the u very fine" 

 light portion gave : 



quartz : feldspar =221 grains : 139 grains — 38*6 

 per cent : 61*4 per cent. 



Weathering of the feldspars. — This is a very important fea- 

 ture in the petrography of a sandstone, but the evidence here 

 does not appear very coherent. 



General appearance of weathering. — On a mere inspection 

 of the sands I was at first struck by the great freshness of 

 many of the feldspars, which were perfectly limpid with fresh 

 faces and sharp angles. 



Ratio of fresh to weathered feldspars. — But if the results 

 of a count of fresh and weathered feldspars are compared with 

 those given by Mackie,* in his study of this feature, the 

 weathering appears rather advanced. Of course such com- 

 parisons may be very misleading on account of differences in 

 ideas of different observers as to what constitutes weathering 

 in feldspar and above all as to the identification of completely 

 weathered feldspars. Following is Mackie's statement of his 

 method of procedure :+ 



" Fragments giving a play of colors in polarized light have 

 been counted as fresh. If they remained turbid or exhibited 

 no play of colors, they have been counted as kaolinized. A 

 third group, usually showing a low percentage of fragments, is 

 marked off as showing intermediate character." 



In this examination, however, only perfectly fresh, limpid 

 feldspars were counted as fresh. As weathered were counted 

 only those showing complete compound polarization.;}: A 

 third group contained those of intermediate character. From 

 the results tabulated below it will be seen that this last differed 

 from Mackie's intermediate group in being on an average 

 about as large as each of the other two. It included all 

 cloudy or impure-looking feldspars and those, not numerous, 

 showing incipient compound polarization. Following Mackie,§ 

 this intermediate group is distributed equally between fresh and 

 weathered feldspars, though personally I should prefer to class 

 it entirely with the weathered. 



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

 of the conditions of contemporaneous climate, Trans. Edinburgh Geol. Soc, 

 pp. 443-468, 1898. 



\ Mackie, loc. cit., pp. 457-458. 



% Here is one of the important factors of uncertainty since other substances 

 than kaolinized feldspar may well have that character. In general, however, 

 the fact that the index of these fragments corresponded about to that of 

 kaolin (1-54) supported the assumption. Furthermore, especially in the 

 coarser sands, many of these grains were quite rectangular in outline, like 

 feldspars, and one of them, separated from the " medium : ' sand, had quite 

 the appearance of a cloudy feldspar grain but was too soft to scratch glass. 



§ Mackie, loc. cit., p. 468. 



