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Gueydan Formation
Note: No. 1. — Typical vesicular Fant tuff, 2% mi. N.W. of Simmons, Live Oak Co.
No. 2. — Light grayish-green, vesicular tuff of the most basic va riety seen in the Fant member, 4 mi. west of Three Rivers, Live Oak Co.
No. 3. — Dense-textured, somewhat silicified, micaceous white tuff probably belonging to the Fant member but possibly Fayette, 6 mi. N.N.W. of Mirando City, Webb Co. High silica percentage is due to presence of secondary silica.
No. 4. — Massive-bedded, friable, calcite-cemented sand-tuff, re sembling sandstone, from Fant member 2% mi. N.W. of Simmons, Live Oak Co. High percentage of CaO is due to calcareous cement.
No. s—Grayish-pink,5 — Grayish-pink, gritty tuff of Soledad member, 4 mi. S.W. of Simmons near McMullen-Live Oak county line.
No. 6. — Light pink, friable, finely pisolitic bentonite or tuff aceous clay from the Chusa member, White Creek 2% mi. S.E. of Simmons.
The SO 3 in these rocks probably is from barite and gyp sum, although very small amounts of these minerals were found when they were analyzed microscopically. Barite veins were noted in a number of these tuffs but the per centage of CaO, with which the BaO was evidently included, is not as high as would be expected from the amount of SO 3 present. Possibly some of the SO 3 is present in soluble iron, aluminum or sodium sulphates which would not be detected in the microscopic analyses of water-washed samples. The principal features brought out by these analyses are the high percentages of combined water, probably mainly in the clay mineral, montmorillonite, to which the glass in the tuffs chiefly alters, and the low percentage of the alkalis, soda, and potash. The much greater proportion of soda than of
Sample No. 1
2
3
4
5
6
SiO 2 55.89% A1 2 0. Fe 2 O 3 16.44 MgO 1.35
57.05% 21.40 1.09
67.13% 11.91 1.42
46.15% 12.02 0.78
58.90% 18.23 1.48
59.38% 16.75 0.43
CaO 0.16
1.64
3.11
15.10
2.51
1.47
Na 2 O 1.03 K 2 O 0.36 SO 3 2.93 Ignition loss 14.43
2.11 0.38 2.35 12.70
2.36 0.28 1.53 11.55
2.87 0.43 2.38 14.75
3.03 1.20 4.03 10.34
2.29 0.56 3.20 13.07
Total 92.59
98.72
99.29
94.48
99.72
107.15
