R. T. Hill—Cross Timbers in Northern Texas. 293 
gin of the Lower Cross Timbers is clearly defined, as is the case 
with the eastern edge of the Upper member; but, instead of 
being below the level of the prairie it is always at a higher alti- 
tude. The soil of the Lower Cross Timbers, although also are- 
naceous, differs from that of the Upper in many respects. It 
is ferruginous and more fertile, averaging less than ninety per 
cent of insoluble silica, while that of the Upper usually exceeds 
ninety-seven. The difference in fertility of the soils produces 
a varietal difference in the flora, the trees attaining much larger 
proportions and the number of species being slightly greater. 
The average width of the Lower Cross Timbers does not exceed 
fifteen miles, and they lose their identity near the Brazos 
River, at Waco. 
It is now evident that the resemblances of the two belts of 
timber are only of a general character, and an examination into 
their geologic relations will show many more important differ- 
ences than those already pointed out. 
Prevalent Theories as to the Cross Timbers. 
The occurrence of these anomalous belts of timbered region, 
extending in a direction at right angles to the streams, has 
been productive of many hypotheses concerning their origin. 
Unfortunately, very few geologists have heretofore visited the 
country, and hence most of these hypotheses have been based 
upon compilation or tradition. Most of them imply that the 
arenaceous soils of the Cross Timbers have resulted from the 
sediments of post-Cretaceous aqueous channels or basins, 
which have been preserved intact from destructive denudation. 
These theories are partially derived from the relative position 
of the timbers to the adjacent prairies. One of them is that 
they represent arms, or inlets, of the Tertiary sea*—a theory 
made plausible by the proximity of their southern termini to 
the western borders of the Tertiary area, and the occurrence in 
one of the Timbers of a molluscan fauna, presenting at the first 
glance a Tertiary facies. A second theory is that they are the 
beds of extinct lakes, and a third, that. they represent the 
channels of Quaternary rivers,t the directions of which indicate 
the former general slope of the surface of the country. The 
current opinions concerning their geologic age have been as 
* “ Hirst Annual Report of the Geological and Agricultural Survey of the State 
of Texas, by S. B. Buckley, State Geologist,” pp. 61, 62. Houston, 1874. 
+ Dr. Loughridge speaks thus of the river channel theory: “In many points 
along its course the general surface of the belt is lower than the adjoining prai- 
ries, and the general features of the lower Cross Timbers almost lead to the con- 
clusion that at some time a deep trough-like valley connected Red (perhaps the 
Canadian) and Brazos rivers, probably forming the bed of the former river and 
conveying its waters to the Gulf through .the present channel of the Brazos, and 
that by some agency this valley was filled with clays and sands and the present 
channel of the river formed.”—Rep. on Cotton Production, p. 29. 
