40 GEOLOGY. 



not prepared to state ; though we think it is possible that some of it may be correctly referred 

 to the coniferae. 



This fossil wood belongs to both of the subdivisions of the green sand belt, and may he 

 ascribed to them as a geological characteristic; it occurs more frequently, however, in the 

 upper portion. It is not only to be found in low places, in the bottom, and on the borders of 

 valleys, but also in the midst of the elevated and vast prairies that stretch from one water- 

 course to another. It here lies scattered broadcast among the pebbles of the diluvial drift that 

 covers all the plains and slopes. 



PHYSICAL FEATURES OF THE COUNTRY 



The section of country embraced by the green sand belt, though somewhat similar in its 

 outlines to the table-lands of the cretaceous limestone above, presents a more gently rmdu- 

 lating surface. Although the upper side of this belt shows the horizontal stratification of the 

 cretaceous system, yet, being interspersed with ranges of hills and ridges of a later age, (and 

 even of tertiary origin,) the land as a consequence becomes more rolling. The substrata of 

 limestone which constitute the beds of the clear water streams between Las Moras and San 

 Felipe being placed much deeper in the lower country, and probably out of contact with the 

 subterranean strata of metamorphic rocks belonging to the volcanic dyke of the country, may 

 account for the general dryness of the green sand region ; for this seems to be entirely deprived 

 of running streams, as elsewhere stated. 



The dry water-courses, sometimes cut from 50 to 80 feet deep, show only at distant intervals 

 pools or ponds. These reservoirs contain a dirty, green, brackish water, on which man with 

 the wild and domestic animals alike have to depend. This water benefits but little the sur- 

 rounding soil, which affords at most only a scant vegetation. Narrow and limited strips only, 

 bordering on the immediate edges of these ponds, flourish with the vegetation of a well watered 

 soil. These water-holes, though, bad as they are, are real oases in this desert region, and 

 afford the only meeting places of animal life ; here, the white man, a traveller, and for the most 

 part jieaceable in his pursuits, the Indian, more or less hostile to civilization and humanity, and 

 the herd-driving, half-breed Mexican, seek this indispensable gift of nature. 



Such places are easily recognized, even at a considerable distance, by the presence of numerous 

 and various flocks of birds, and a nearer approach shows them to be marked with a denser 

 growth of trees, shrubs, and weeds. They constitute a distinct characteristic of these regions. 



Between the densely covered borders of these watering places and the bare slopes and arid 

 heights, dells, basins, and all other varied depressions, almost always exhibit a more luxuriant 

 state of vegetable life. This condition may be explained in part by the increased hygrometric 

 capacity of the atmosphere, however little moisture may be deposited. Here, also, the rain- 

 water sweeps together the more fertile particles of the soil, which, possessing a great deal of 

 moisture, comparatively, causes vegetation to spring forth. In this same belt of country, on the 

 Mexican side of the river, there are more running streams than on the Texan side ; and for the 

 reason that the conditions on which their origin and existence depend, are less distant, as will 

 be made evident from what follows. 



