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in contour than iu the recess into which the Cimarroucito opens out on 

 leaving its picturesque mountain-hemmed, valley. Here, in consequence 

 of the rapid pitch of the valley, the terraces have a more marked incli- 

 nation with the stream than is observed on the Cimarron, and where 

 they present several distinct levels, often isolated, forming low drift- 

 covered mesas or wide slopes rising upon the flank of Urac Mountain. 



Besides these more strictly flu viatile formations, in places the borders of 

 the basin are marked by low benches of considerable extent, and which are 

 distinguishable from those occurring immediately along the water-courses 

 by their more level surfaces. Between the Eayado and Cimarron, as also 

 between the streams to the north of the latter, these benches form sev- 

 eral levels connected with the main slope flanking the Tertiary plateau, 

 and which are usually distinctly defined by the abrupt descent facing the 

 plain, as shown in the sketch of the Baldy range and that of the Tanaja 

 Mountains. Their tops are covered by a thin sheet of fine drift-material, 

 or the soft Cretaceous shales constitute the subsoil. At other places, lim- 

 ited tracts of barrens are encountered, where deposits of loose sand have 

 accumulated, which the winds mold into ever-varying miniature downs, 

 and whose vegetation, if not peculiar, is distinguished by the prevalence 

 of cacti and those plants which thrive in a sterile soil. 



The latter system of terraces probably are more properly referable 

 to an earlier time, when the drainage of the basin was efl'ected; while the 

 modifications resulting in their present conformation is as probably due 

 to the combined erosive action of the Canadian and its numerous afflu- 

 ents, when their volume of waters, issuing from the vast reservoirs in the 

 mountains, was far greater than at the present time. 



The nature of the loose materials covering the terraces in the mouths 

 of the valleys plainly indicates the source of their derivation in the 

 mountains to the west, and which were brought down at a period ante- 

 cedent to the time when the streams had deepened their beds to their 

 present level. The coarser materials and bowlders (the latter generally 

 of small size) largely consist of granitic and gneissic rocks, with a variety 

 of igneous and metamorphic material, whose parent ledges, from which 

 they were torn during the Glacial epoch, are met with in the mountains 

 a few miles distant. But these plains accumulations are insignificant 

 compared with the vast deposits which fill some of the park-like valleys 

 in the heart of the mountains, and which rise high upon the environing 

 declivities. 



Besides this comparatively thin sheet of drift-material, the plain is 

 here and there occupied by low butt6s and mesas of limited extent, 

 remnants of the Cretaceous shales, the summits of which are often 

 covered by a sort of concrete, consisting of coarse gravel cemented with 

 lime. One of these outcrops occurs on the isummit of the elevation near 

 Mr. Arms's, forming a coping to the little mesa upward of three feet in 

 thickness, its presence serving to protect the soft underlying shales from 

 atmospheric erosion. Similar deposits are met on the lower course of 

 the PoQil, a tributary of the Cimarron, where the outcrop appears in 

 the low blufl'-banks, at a level possibly not much higher than the expos- 

 ure on the Canadian just mentioned. It is evident that this concrete 

 deposit is of quite modern origin, but its relations to the drift and late 

 Tertiary deposits are not so clear. It would, however, appear to ante- 

 date the drift, the modified strata of which its comparatively limited 

 extent recalls, from the fact of its occurrence on the tops of the outly- 

 ing remnants of the Cretaceous in the midst of the basin; while it is. 

 undoubtedly of much more recent origin than the sandstones at the 

 base of theLignitic formation, from which it widely differs in the char- 



