14" Till AiiH rlciin Geologt)<t. Seiitember, 1891 
that of the lianlcr rocks of IIk' Cliitiuilas Ihoiv arc often im- 
portant mineral deposits. 
2. lln fornuttiiin i)f thrphtln. Tlic formation of tlie valley of 
the Sabinas (»r j)lain which extends northward from the Santa 
Rosas is entirely different from that of the mountains, and is 
composed ( a ) of thin, laminated, crumbling, calcareous, arena- 
ceous clays, alternating with strata of thin sand and limestone 
accompanied by numerous coal beds, lignite ]>eds, silicified trees, 
fossil molluscs and occasional bones of animals. This is the 
great coal formation of the Sabinas and l)elongs geologicall}' to 
the very latest epoch of the Upper Cretaceous, or Glauconitic 
Division, and should not be confounded with the lignite beds of 
the Tertiar}' period of tlie eastern United States. Init is allied to 
the coal beds of the Rocky Mountain region as worked in New 
Mexico. Colorado. Wyoming and Utah, and which is one of the 
most valuable and important fuel-producing terranes in the world. 
This coal field is the onl}- one developed in Mexico, and at the 
mines at San Felipe some twenty miles distant, the coal is ob- 
tained for all the railwa}* systems of Mexico, and largely ex- 
ported into the United States. These rocks are sub-horizontal a 
few miles from the mountains, but at their contact with them 
they are uptilted. showing their participation in the uplift, and 
that the age of the mountains is Post Cretaceous. 
Before the erosion which exposed these foundation measures in 
the plain, the}' were covered by two later and ditferent formations, 
which will next be explained. The whole Rio Grande embay- 
ment is underlaid by this coal formation which is more fully dis- 
cu.ssed later. 
(h) Till YiiJUij Conglomerate or Terraces. Forming a terrace 
or bench of the plain for several miles awa}' from the mountains 
and extending u\) the canons to a certain level, there is a great 
sheet of conglomerate lying horizontally and composed of large 
rounded pebbles of the mountain limestone, cemented b}' a cal- 
careous matrix. This conglomerate, as exposed by the cutting of 
the arroj'os (dry creeks) and canons, is over 100 feet in thick- 
ness and is the shore deposit of the great sheet of water, which 
in late Quaternary times extended over millions of square miles of 
western North America, and which is one of the most remarkable 
features of the continent. 
These terraces of the Santa Rosa have their counterpart on the 
