26 GULF TERTIARY OF TEXAS. 



the Basal Clays of the Tertiary period, already described, are met and extend 

 thence for some seven miles to within two miles of Pond Creek. About a 

 mile and a half below Pond Creek is seen an outcrop of Tertiary sand, con- 

 taining black specks and rendered plastic by a white clay. It is capped by 

 semi-indurated Quaternary gravel and sand, and contains large nodules 

 which give a strong reaction for carbonate of lime, and which are simply 

 hardened masses of the enveloping sand. They are one to eight feet in di- 

 ameter, hard, kidney-shaped, flat or nodular, and project out of the compact 

 sandy bluff in a most characteristic manner. Loose fragments of silicified 

 wood, which have also doubtless been derived from the same bed, lie among 

 the many nodules that have been eroded out. So many of these rocky 

 masses have been loosened from the sand and piled up in the bed of the 

 river that they have obstructed its course, and have formed rapids. Many 

 of these rocks are round or oval, and are locally known as "kettle bottoms." 

 Such strata as these are seen down the river for a mile and a half from this 

 point, where they dip under a series of gray clays containing beds of lignite, 

 varying from one to five feet thick and associated with ferruginous sand. 

 The clays contain large masses of silicified wood, which is sometimes seen in 

 places in the bed, but more often has been weathered out and lies in the bed 

 of the stream. Occasionally nodules of clay ironstone, generally in a semi- 

 oxidized condition, are found. Such strata are exposed for about a mile, 

 when the gray sands with calcareous concretions and indurations again ap- 

 pear. This deposit contains considerable quantities of iron pyrites, and the 

 indurations are often cut by veins of crystalline calcite. A short distance 

 below here is Calvert Bluff, Eobertson County, where lignite occurs in large 

 quantities and has been worked intermittently for many years. The beds of 

 this strata are shown in the following section: 



1. Brown and red river silt 10 feet. 



2. Gray clay ... to 3 feet. 



3. Lignite 12 feet. 



4. Gray clay 2 feet. 



5. Lignite 2 feet. 



6. Gray clay 3 feet. 



The clay beds in the above section contain large clay ironstone concretions, 

 which enclose many leaf impressions. The lignite is black, woody, friable, 

 and of a dull lustre. It is faulted and much jointed. D|p, three degrees south- 

 east. From here to where the International and Great Northern Railroad 

 crosses the river we see sand beds with calcareous indurations, such as have 

 been described at Rocky Rapids. At this point is a bluff showing sixteen 

 feet of Tertiary strata, capped by over fifteen feet of a highly calcareous light 

 green and yellow Quaternary clay containing many small white concretions. 



