Geology of Northern Mexico. — HW. 141 
northern side of the Rio Grande embaj'ment in the great gravel 
deliris and estuarine deposits of that region. 
In some of the mines near tlie contact of this conglomerate 
and the Chiquitas it forms an important element in excavating 
and drifting. 
(c) Tlic Basaltic or volcanic mesas. Next to the mountains 
proper the most conspicuous feature of the region is an elongated 
tableland or mesa and a few disconnected buttes, which extend 
northward from the base of the range north of the Chiquitas 
out into the plain. The summit or table of these is 
composed of hard black volcanic material, the remnants 
of a great lava flow which once covered much of the plain, 
but most of which has since been destroyed by erosion. 
This lava sheet is not over 100 feet in thickness, and 
extended up to and against the Chiquitas covering the coal fields 
unconformably, and having been a most important factor in pro- 
ducing the mineralogical conditions of the area. By its weight 
and heat the lignites of the Sal)inas coal fields were converted 
into the superl) coals, and the metallic fillings of the veins and 
fissures are apparently conuected with it. Far out into the plain 
towards San Felipe, remnants of this lava flow can be seen, 
although the source from which it came is unknown, for there are 
neither craters, fissures, dikes, intrusions or other vents visible in 
the Santa Rosas ( although they have l)een erroneousl}' reported ) 
and the nearest known to the writer are in Uvalde and adjacent 
counties of Texas 150 miles distant. Whatever may have 
been the source of this lava there can be little doubt, from the 
occurrence of the valual)le minerals onh' in those Chiquitas 
in its vicinity, that the origin of the minerals is closely con- 
nected with its phenomena in comparatively recent geological 
time. 
The Santa Rosas are surrounded on all sides by this plain, and 
completely disconnected from the other mountain blocks seen to 
the northwest and east which are all of the same general type of 
structure, i.e., isolated mountain blocks surrounded liy plains. 
Many of the other ranges have porpliyritic and l)asaltic extrusions, 
but the Santa Rosa pr()[)er is merely a remnantal block of the 
('retaceous rock sheets, broken and faulted. It is the simplest of 
all the mountain blocks of the Basin region, and hence I have se- 
lected it for descrii)tion. 
