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As a primary fact established by the work of this survey, it may be stated that Texas is a geological centre or focus. Situated as she is at the centre of the southern border of the United States, the converging lines of the geologic formations which mark the eastern ocean and gulf coast, as well as those of the central basin and western mountain system, all meet within her borders. Here it is that the true relations of each to the other may be best ascertained, and it will require the facts, that will only be brought to light by detailed study of these relations as they do exist here, to properly explain and interpret much that is now in doubt in the regions further north.

These various converging areas may be said to divide Texas into four districts.

The first of these, which is a continuation of those formations bordering the Gulf shores of the States east of us, comprises that portion of Texas lying between the Gulf coast and the foot of the Grand Prairie region. In no place does the altitude of this broad belt exceed 700 feet above the sea level. Beginning at the coast in level prairies, it gradually becomes undulating, and then hilly, until it meets the higher hills which form the scarp bounding it on the north and west.

From this boundary line to the foot of the Staked Plains is a second district which stretches away to the Rio Grande and Pecos of the south and west. This district may be subdivided into a plateau on the east and south called the Grand Prairie and an interior basin. This also rises gradually towards its western border, and, though containing many hills and mountains, nowhere is there an altitude of over 2500 feet. Within the encircling rocks of more recent deposition, there lies within this district, like an island in the sea, the older nucleus of the Llano or Central Mineral District, which can well claim close relationship with the oldest rocks which have been found upon our continent.

The Staked Plains, which form the third district, rise from 300 to 400 feet higher, and are a part of the great plains which stretch eastward from the foot of the Rocky Mountains. These plains, while in reality they rise rapidly towards the west, seem almost like a level floor,