Geology of Xorfh em Mexico. — /////. 137 
in that they and their foot hills (the lomitos) contain the valuable 
mineral deposits, (e) Los Lomitos. Northward and parallel to 
the Chiquitas — just as the Chiquitas are parallel to the Sierra, 
and in the same manner as they subtend the Sierra, is a row of 
low. rounded, grass-clad hills known as Los Lomitos (the little 
hills). These are ver}' small, usually not over 100 feet high, Ijut 
are an important feature of the district. 
2. The Plain. Looking from the mountains north and east- 
ward can be seen an extensive stretch of sul)-level country — the 
•basin of the Rio Grande and Sabinas — the great coal field of 
noithern [Mexico. There are several features of this plain which 
deserve brief mention however: (a) the valley of the Rio Sabinas, 
which is from 12 to lo miles north of the mountains, a beautiful 
stream of great volume, which has worn its way down through 
the strata to a distance of about 1,000 feet below the base of the 
mountain, therebv exposing the geological structure of the plain. 
Remnantal of this great erosion of the Sabinas are several import- 
ant topographic features: (b) the volcanic mesas, which when 
"viewed from the west, resemble an elongated platform or liench 
projecting northward from the Ijase of the Chiquitas. Its top is 
perfectly level, liut the precipice which surrounds it is about 100 
feet high, (c) Near the northern end of this ta1)le land are sev- 
eral disconnected flat-top circular hills (buttes) which are rem- 
nant of the former extent of the mesa. 
(d) TIk LntiKis. At several places in the A'alley are low hills 
of yellow sands and clays, the remnants of the valley of the Sabi- 
nas, which has cut down into the coal measures constituting the 
foundation of the plain, upon which the conglomerate and lava 
have been deposited. To the northwest and across the river these 
lomas (hills) have considerable extent and form a low range of 
elevation a))out 30(1 feet above the river. 
(e) T/ie (hngloincrate Ttri-Kcr. Xear the base of the mount- 
ain, and extending into the valle}' from one to four miles is a 
level bench composed of fragments of mountain rock. "Wher- 
ever this ])ench or terrace is eroded, beneath it are found the soils 
used by the agriculturists, irrigated by the streams from the 
mountains. No doubt this formation once covereil most of 
the plain. 
III. (iKilI.OOir STRl'CTl'RE OF TUK KE({IO\. 
{<() Tin iiirn'ii iitnss (if inniuitdlii nr Siirrn Gnoidf is composed 
