INTRODUCTORY. 



The rocks which underlie the upper part of the Rio Grande 

 Embayment, belong to the Cretaceous and the Tertiary- sj'stems. 

 They consist of limestones, sandstones, clays and marls. In a few 

 very limited localities dark porphyry has been injected between 

 these stratified formations. The rocks have suffered but little 

 disturbance. Generally speaking, we find them in the position 

 they were orginally deposited on the bottom of the ancient wa- 

 ters, excepting that they have been elevated toward the north- 

 west and have sunk donm toward the southeast. The general 

 tilting in that direction amounts to about forty or fifty feet to 

 the mile. 



By the process of general erosion and land waste, these forma- 

 tions have been cut down so that the land now presents a quite 

 even plain, whose surface has a general slope in the same direc- 

 tion as the beds dip. For the whole area this slope averages 

 less than ten feet to the mile. 



As a result of this general erosion and tilting we now find the 

 lowermost and oldest strata exposed to the northwest and the 

 uppermost and more recently deposited beds appear farthest 

 south and east. 



From Devil's river to the Dos Hermanos country there are ex- 

 posed some 4,i00 feet of strata. In the following report I shall 

 describe these, beginning with the oldest formation, giving in full 

 my observations on their mineral character, the surface distribu- 

 tion, and underground extent. (See map). 



