FRUM THE h'ECOS UOWNWAKD. 29 



gray limestone. This formation, viewed in its lithologic character, may be considered as car- 

 boniferous ; yet the frequent occurrence of fossil remains within its limits hardly admits of its 

 separation from the cretaceous system. 



The outside of this limestone is, as already stated, of a dark ash color, often rough, pre- 

 presenting the appearance that a violently boiling mud-pool would after being upheaved and 

 suddenly cooled. Its inside is often white or pale yellow, and mealy, with a great tendency to 

 disintegration, which causes a great many holes, fissures, and excavations of every shape and 

 description. These give this limestone a peculiar appearance, and one that is remarked by every 

 traveller. 



VALLEYS. 



The small, as also the larger, valleys are mostly formed by the continued washing out of the 

 dells and fissures. Thus formed by denudation throughout the whole country, with their 

 borders cracked in every direction, they deserve only the name of deeply cut ravines, (carions.) 

 This cracked peculiarity may be ascribed to the combined influence of a high temperature, to 

 which this formation may have been at some time exposed, and a subsequent more or less 

 gradual refrigeration. There is scarcely a doubt but that all these table-lands were also formed 

 under the sea, and at the same time exposed to volcanic action. If so, this limestone really 

 deserves the name metamorphic, and its somewhat anomalous appearance would be accounted for. 



It is our opinion that the limestone of the region above referred to is not of the same character 

 throughout ; it is not uniform, and appears under the most variable shapes. It may be seen in 

 various localities, alternating with strata that bear the most striking resemblance to magnesian 

 limestone. 



STRATIFICATION. 



The strata of this formation are generally arranged horizontally ; sometimes, however, local 

 disturbances appear which placed them into synclinal or anticlinal positions. The lower strata, 

 often being of less solidity than the upper, and readily disintegrating under atmospheric 

 agencies, are finally washed out into excavations by the action of water. These excavations 

 occur commonly in the beds of ravines, and also in the banks of the river as considerable caves. 

 They are also to be seen near the top of the table-lands and hill-ranges, lying as so many 

 terraces, one above the other ; the more solid layers, resisting the action of external agencies, 

 project far beyond the softer. 



High table-lands, intersected with deep vertically walled valleys, characterize the face of the 

 whole country. The walls of these valleys, or more properly speaking, canons, are variously 

 cracked open, and presenting ravines of greater or less extent in all directions. 



Those valleys seem to have relation only to the lithologic character of the formation. They 

 are, therefore, usually short, and do not terminate in gradually diminishing fissures, like the 

 heads of rivers and creeks, but suddenl]* end with a deep chasm under a vertical wall of rock. 

 Deep holes are washed out under these masses of rock, where rain water collects and remains 

 for a considerable time. 



WATER HOLES. 



Excavations similar to those here mentioned, and retaining water for some time, occur also in 

 the usually dry beds of the tributaries of the Rio Bravo. This is in most cases the only water 



