F. GEOGRAPHY. 265 



rapid erosions and ready disintegration of the shales belong- 

 ing to Cretaceous No. 2. Springs containing an unusual 

 amount of mineral ingredients, some of them hot, occur. in 

 these valleys. Owing to the slight southerly dip of the 

 cretaceous beds, this formation claims a considerable area 

 of the region, extending from the Rio Animas eastward to 

 the border of the district. Above the well- determined 

 strata of Nos. 2 and 3, a series of shales and sandstones set 

 in, in which no characteristic fossils whatever were found. 

 They reach a thickness of about three thousand feet, and 

 contain coal at a number of points. It will not be possible 

 to determine their geological age with any degree of cer- 

 tainty, until careful comparison of the parallel formations 

 observed by Mr. Holmes and Dr. Peale can be made. The 

 absence of fossils is greatly to be regretted; but none were 

 found, although many square miles were traversed contain- 

 ing the series. Speaking with the reserve that imperfect 

 comparison of the notes taken dictates, it would appear that 

 the Trinidad coal-bearing series is parallel to this one. 



After having completed the survey of this lower region 

 along the Rio San Juan and its tributaries, the work was 

 continued to the extension of the La Plata Mountains. 

 Here again volcanic rocks were met with, identical in every 

 respect with those farther north and west. Here, as w r ell as 

 previously on the headwaters of the Pinos and Piedra, evi- 

 dence of former glaciers w T as found. Considerable areas 

 showed the grooving and striation of rocks in positu, pro- 

 duced by the motion of ice and boulders. Deep canons were 

 cut into the volcanic conglomerate occurring there, that had 

 not preserved grooving and striation however, owing to the 

 rapidity with which it yields to the effect of atmospheric in- 

 fluence. A gentle slope eastward of the volcanic rocks, that 

 there reached to the youngest member of the group basalt 

 gradually merged into the San Luis Valley. Affected by 

 local basaltic eruptions, as well as by the easterly dip of 

 the volcanic beds, the drainage on the west side of this val- 

 ley presents some interesting features, consisting in sudden 

 curves northward. Northward, the unbroken flows of ba- 

 salt continue on the west side of the valley until Rio Ala- 

 mosa is reached, where they end and drift begins. A num- 

 ber of volcanic bluffs, trending nearly north and south, sep- 



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