LOWER RIO BRAVO. 33 



PARALLELISM AND VOLCANIC CROSS AXES OF THE MEXICAN CORDILLERAS. 



By this parallelism is meant an inclination of the chief sierras to separate into collateral sub- 

 sierras and side branches, which again join the main chain ; or they are at least connected 

 either by cross sierras, or even simple dykes. To this striking peculiarity the profiles of both 

 the northern and southern portions of the western hemisphere are due. This mountain 

 range of Santa Kosa presents another characteristic, and one, according to Alexander von 

 Humboldt, peculiar to the mountain system of Mexico — it is, that the volcanic axes cross the 

 direction of the Cordilleras almost always at right angles. There are many cross- valleys on 

 the northeastern slope, proving the action of some volcanic disturbance ; a valley some eight or 

 nine miles east of Santa Rosa, called " El Potrero," is the most remarkable instance. Several 

 mines are still worked here, where the metalliferous limestone is variously traversed by veins of 

 feldspar and limestone spar ; this latter usually accompanies the silver ore and galena. In the 

 centre of this cul de sac, a better name for this so-called valley, can be seen an ancient crater, 

 the inner walls of which are thickly coated over with a lava-like basalt of a dark red hue, whose 

 composition differs apparently but little from that which covers in layers (20 or 30 feet thick) 

 the cretaceous range of hills joining the northeastern slope of the metalliferous mountains. 



It may, perhaps, be of importance to state that the argentiferous portion of the Sierra de Santa 

 Eosa is not more than ten or eleven miles in length, commencing at a point called ' ' El Cederal , " . 

 (the Cedars, Cedar Grove,) and terminating at a place, in a northwest direction, bearing the 

 name of "Los Nogales," (the Walnuts, Walnut Grove.) The presence of ores, together with 

 the trap or basaltic dykes branching out from the sierra at right angles, may prove the suppo- 

 sition which places here the origin of the volcanic power, that, pushing through the fissures of 

 stratified rocks, caus^ the dyke before alluded to. Following this dyke from Santa Rosa up 

 towards the Rio Bravo, it will be found to cross the valley of this river in the vicinity of Elm 

 creek, as already stated. 



BASALTIC HILLS— VOLCANIC DYKES IN TEXAS. 



On the Texan side, the first marks of volcanic action are to be seen at the head of Leona river. 

 Here a solitary hill of 60 to TO feet in height occurs, formed entirely of a dark green basalt 

 which is closely allied to that of the Santa Rosa mountains, and which also contains much horn- 

 hlende and olivine. In the vicinity of Fort Inge, and also near the head of Las Moras, are sev- 

 eral hills of the same nature ; also the road from Leona to the first crossing of Devil's river 

 leads over several places indicating volcanic action. 



The west bank of the Rio Frio, at the crossing, is formed of a solid mass of basaltic rock; 

 which undoubtedly belongs to the dyke alluded to as having its origin in the Santa Rosa moun- 

 tain, and here crossing the cretaceous formation. 



DE. EOEMER'S VIEWa 



Dr. Roemer, in his not yet translated work, entitled "Die Kreidebildungvon Texas," ("The 



Cretaceous Formation of Texas,") mentions (page 8) that plutonic or volcanic rocks were brought 



to him from between the San Saba and Cibolo; and according to him, granite, together with 



older stratified rock, is seen in narrow strips, surrounded with cretaceous strata, between the 



5 M 



