16 GULF TERTIARY OF TEXAS. 



could be produced by the evident estuary character of many of these 

 beds. 



5. The fossil remains are frequently much broken, worn, and rounded, as if 



by continued rolling on or near a sea beach. 



6. Frequently the fossils are found in great beds of solid shells, sometimes 



as much as ten feet thick, a condition of things that strongly denotes 

 littoral conditions, and that is seen going on along our coast at the pres- 

 ent day. Such beds of fossils are seen on the Rio Grande in many places 

 between Laredo and Brownsville. 

 The Tertiary strata strike in a general northeast and southwest direction, 

 approximately coincident with the coast, and dip gently toward the east or 

 southeast at an angle varying from to 5 degrees. This dip, however, is 

 very irregular and undulating, and no estimates of thickness of strata based 

 on it can be relied on. In fact, a northerly or northeasterly dip is of no un- 

 common occurrence, though it is simply a local phenomenon. On the Rio 

 Grande the strata all dip normally to the east and southeast, until we get to 

 a point ten miles above San Ygnacio, when the dip changes to northeast at 

 an angle of 1 to 8 degrees, and retains this direction for a distance of twenty- 

 one miles down the river. Similar occurrences are seen elsewhere, though 

 in no other place were the strata observed to maintain this dip for so great 

 a distance. Usually a northerly dip returns to its normal direction in a dis- 

 tance of a few hundred yards at the most, though a horizontal and undu- 

 lating dip often extends for many miles. This variable character of the dip, 

 however, does not require the supposition of a disturbance or upheaval of the 

 strata for its explanation. It is doubtless due to the natural sinking and 

 warping in a great thickness of soft beds. In fact, it would seem a most un- 

 natural thing to see several hundred feet of soft clays and sands, covering 

 an area of many thousand square miles, lie horizontally when they were ex- 

 posed to the influence of atmospheric agencies. The unequal expansion and 

 contraction of strata of different constituents, due not only to heat but to the 

 drying out of the beds, would alone account for much or all of the warping 

 that is exhibited throughout the Tertiary country. Besides, the chemical 

 action that has gone on in these beds is probably also accountable for a part 

 of the variable dip. Faults are of frequent occurrence, and are to be ac- 

 counted for on the same principle as the variations of dip. They are rarely 

 over eight or ten feet in throw, and play no important part in the features 

 of the country. One of the most clearly defined faults seen is represented 

 in figure 2. Jointing is also a very common phenomenon throughout the 

 whole of the East Texas region. 



Estimations of thickness of the Tertiary strata of this region are attended 

 by peculiar difficulties, as the dip is too variable to be relied on in such esti- 



