276 M. 1. Goldman — Catahoula Sandstone of Texas. 



Ratio of different species of feldspars to each other. — 

 Mackie * gives as the order of weathering of the three main 

 types of feldspars : plagioclase, orthoclase, microcline. The only 

 quantitative data for their relation that I know of are in Thoulet's 

 study of the Gulf of Lyonf and a few scattered through the 

 volume of his pupil Sudry on the Lagoon of Thau, f These fig- 

 ures are only approximate, having in some cases a range as great 

 as 50 per cent, but indeed the great variability of the ratio of 

 plagioclase to orthoclase (microcline is not counted separately) 

 justifies this rough counting. From an inspection of Thoulet's 

 "table it appears that in general the plagioclase is less 

 abundant, but the great variability in the relative amounts 

 suggests that this fact is to be ascribed largely to the original 

 proportions in the source rock. It will be convenient to discuss 

 this feature of the Catahoula sandstone more in detail in the 

 following discussion of the heavy minerals. 



5. Ratio of heavy to light minerals. 



Following Thoulet, the ratio of heavy to light minerals (feld- 

 spar and quartz) has been determined for the " very fine " sands 

 only;;}; but this suffices since the most available data for com- 

 parison are those given by Thoulet. Taking up first ordinary 

 marine coastal deposits, I find that the average of 15 analyses 

 selected at random from Thoulet's paper on the Gulf of Lyong 

 gives 12*4 per cent of heavy minerals in the " very fine " sand 

 while the lowest thus obtained is 1*1 per cent and the highest 

 26*6 per cent. Inspection of the table discloses a ratio as low 

 as 0*23 per cent, but the very low ratios occur in sands with 

 almost no " coarse," " medium," or " fine " sand ; " very fine " 

 and " extra fine " with " clay " making up almost the whole 

 sample. Turning to another type of deposit, an analysis by 

 Thoulet I of a Sahara sand shows the proportion of heavy 

 mineral in the very fine portion to be about 0'25 per cent.lf 



* Feldspars in sedimentary rocks, p. 448. f Op. cit. 



\ By an oversight the determination was also made for the "fine " sands and 

 gave about - l per cent, showing that as usnal there is a much larger pro- 

 portion of heavy minerals in the " very fine " sands than in coarser sizes. 

 In the " extra fine " sand it is not practicable to make this separation by 

 ordinary methods on account of the great fineness of the material. 



§ Thoulet, J. : Etude bathylithologique des cotes du golf e du Lion. Annales 

 de l'lnst. Oceanogr., Monaco, iv, fasc. 6, pp. 53-60, 1912. 



I Thoulet, J. : Etude mineralogique d'un sable du Sahara, Bull. Soc. Min. 

 Fr., iv, p. 266, 1881. 



^[ An obstacle to direct comparison with this analysis is the absence of any 

 portion corresponding exactly to the " very fine " sand, as " very fine " and 

 "extra fine" together are about represented by "e" which includes all 

 material below 21 mm diameter. But as this portion is only 5 per cent it 

 may be assumed that the amount of " extra fine " material in it is negligible. 

 It is, moreover, noteworthy that in this sand the amount of heavy mineral is 

 about the same in each portion. 



