52 



GEOLOGY. 



singular character of formation has given the name of "Bofecillos" to some of the more striking 

 mountain ledges. 



Owing to the crumbling nature of this underlying stratum, we find it variously washed and 

 often fantastically shaped by the peculiarities connected with its various exposures ; it thus fre- 

 quently forms burrowing caverns and dark grottos, set off with misshapen pillars. Quite inva- 

 riably we find its base occupied by a talus, derived from the overhanging rock, forming a rough 

 slope strewn with irregular blocks ; thus its connexion with the underlying rock stratum. This 

 connexion is, however, occasionally brought to light, and shows a lower develoi^ment of lava 

 rock, differing but little from that above, except in a greater closeness of texture, the upper 

 capping being generally vesicular, while that below is compact. 



Frequently, however, a direct connexion between the separate lava strata is made by narrow 

 dykes, identical in character with the rock above and below, passing through the intervening 

 breccia. At other places, veins are seen shooting from above and below_, and terminating in 

 the intervening series. 



SECTIONAI. VIEW, SHOWING A SERIES OF VOLCANIC rBODCCTS, TKATEBSED BY INJECTED IGNEOUS VEINS ; CANON OF BIG BKAVO, 



TUIBTY-FIVE MILES BELOW PBESIBIO DEL NOBTE. 





A. Dark-colored vesicular trap. 



B. Volcanic breccia of a liglitish brown color in horizontal strata. 



C. Lava or trap-rock of close texture, dark-colored. 



D. Breccia as above, having a light greenish color. 



E. Igneous veins. 



We are now sufficiently prepared to appreciate the external features of the region thus char- 

 acterized. We can understand how the unequal development of these several layers may give 

 shape and character to the mountain ranges, and what diversified features they will necessarily 

 assume, under the influence of denuding causes, acting so unequally on their separate members. 

 The geological formation seems to conspire with the atmospheric influences to give a ruggedness 

 and character of desolation to this region, of which description can give but a most meagre idea. 



It is, however, in the line of the river-course that these rugged features present their grandest 

 developments. We see the turbid waters of the Rio Bravo here contracted to a narrow channel, 

 barely a stone's throw across, sweeping on a resistless current beneath bristling crags ; now 

 tumbling over foaming rapids, connected with some abrupt turn in the course of the stream, 

 and then gliding smooth and unbroken through mountain clefts with perpendicular walls on 

 either hand, rising to the dizzy height of 1,200 to 1,500 feet perpendicular. 



In our progress through the range, the breccia deposit becomes less developed, and finally 

 disappears altogether, or is seen only in narrow seams along the sides of the mountains. 



The course of the river at first is not entirely hemmed in by abrupt rocky walls, a rough 

 talus at the base affording a rude pathway, occasionally lined by narrow strips of sand beach. 



