GEOLOGY. 



The rock exposures on the river bank exhibit disturbed strata of limestone, characterized by 

 frequent fossils as belonging to the Cretaceous i^eriod. A greater or less metamorphism of this 

 or a more ancient sedimentary rock is also exhibited, while on either side of the river tower up 

 to a height of 500 to 1,000 feet rugged igneous rocks, having a granitic texture, and character- 

 ized by Professor Hall as "feldspathic or granitic lava." (No. 40.) 



View of the Initial Point, on Rio Bravo. 



On the Mexican side, the various formations, stratified and igneous, are blended and intermixed 

 in great confusion, the connexion between the various formations being obscured by the irregular 

 exposure of igneous products, the greater or less degree of metamorphism of adjoining sedi- 

 mentary rocks, and the presence of extensive diluvial deposits. The general surface is thus 

 rendered extremely rugged and broken, the traversed roads being obliged to make a considera- 

 ble detour from the course of the river. 



On the American side is conspicuous a high mountain range, nearly parallel to the river, at 

 a variable distance of from 3 to 10 miles, and observing a regular north and south course. This 

 range is seen to be composed of stratified limestone, dipping very uniformly at an angle of 45° 

 W.S.W., or toward the river ; in the face of this dip rest the various igneous outbursts, asso- 

 ciated with the disturbed cretaceous beds. This limestone is determined by Professor Hall to 

 belong to the carboniferous series, being a northern continuation of that before noticed at Eagle 

 Springs. 



The remarkable character of the stratification is conspicuous at a great distance, its deep 

 gullies presenting fine sectional views, and the difierent exposures of its sloping surface exhibit- 

 ing variously curved lines, as the strata are thus brought to view by the action of the denuding 

 forces. 



The highest summit of this range presents a sharp, jagged crest, such as might readily be 



