88 GULF TERTIARY OF TEXAS. 



ating the glauconite. Sometimes, also, finely disseminated carbonate of lime 

 is the cementing material in such rock. 



Occasionally there is found in the East Texas region a hard white sand- 

 stone, often approaching quartzite in consistency, and exceedingly tough. In 

 fact, it is so tough that it has not yet been put to any practical use by the 

 farmers in the neighborhood, as they are unable to handle it with the imple- 

 ments at their disposal; but it is a beautiful stone, and if properly quarried 

 would be a very valuable one. The largest deposit of this seen is on the old 

 San Antonio road, two miles southeast of the town of Alto, in Cherokee 

 County. The road skirts it or goes over it for about a mile. It varies in 

 thickness from one to four feet. Its surface is pierced by holes, often run- 

 ning down for several inches and varying in diameter from one to two or 

 more inches. They are often coated with a rusty crust, and have probably 

 been formed by the decomposition and removal of iron pyrites. This pres- 

 ence of iron pyrites also accounts for the rusty crust and for the occasional 

 rusty specks found in the interior of the rock. It directly overlies the fossil- 

 iferous greensand of the region, and possibly occupies the same horizon as 

 the brown laminated ore elsewhere in the county. Similar rocks are seen 

 three miles west of McKee's Mill, where it is from one to two feet thick and 

 covers a small area, and a variety of it from four miles west of Jacksonville 

 is said to have been very successfully used as a millstone. About five miles 

 northeast of Rusk, on the Conway survey, is seen a similar stone, three feet 

 in the thickest part, and covering a small area of only a few square yards. 

 Here it is associated with sandy and clayey strata about 150 feet below the 

 level of the greensand bed. This exposure shows that the white sandstone 

 is not confined to the horizon occupied by the Alto bed, i. e., directly over 

 the greensand. The origin of this sandstone, which in its hardness and 

 toughness is in such contrast to the other East Texas deposits, is to be found 

 in the induration of beds of clayey sand by silicic acid solutions. Many of 

 these beds become converted to a soft friable rock, even when dried alone by 

 exposure to air, and the infiltration into them of surface waters containing 

 silicic acid would add not only toughness, but also the semi-quartzite appear- 

 ance often seen. The fact that many of the surface waters in the sandy re- 

 gion of East Texas contain silicic acid explains the source of this cementing 

 material in the sandstone. 



On the Rio Grande, the same beds that are soft and incoherent to the north- 

 east have become hardened into friable sandstone, and sometimes, though 

 rarely, into tough siliceous rock. The cause of this greater amount of indu- 

 ration in this region is due to two causes, one climatic, and one chemical. 



1. In the dry hot climate of the Rio Grande there is a greater tendency 



