ENDLicH.] GEOLOGY SAN LUIS DISTEICT, SECTION A. 317 



ervation, and forms the lowest stratum of the Cretaceous. It is Creta- 

 ceous Fo. 1. At Canyon City, the sequence of the strata can be well seen 

 by ascending- one of the bluffs north of the Soda Spring, and looking 

 eastward. 



Eetaining the same curves that are followed by the underlying strata, 

 the Cretaceous skirts the east and southeast slope of the mountains 

 throughout section «, dipping more steeply near them, approaching the 

 level toward the plains. At Oil Creek it forms a bay of about eleven 

 miles in length and from two to six miles in width 5 another smaller one, 

 of about three to four miles in length, is found a short distance west of 

 Oil Creek. More gently rolling surface characterizes these bays with 

 their low bluffs. West of station II another bay, about as long as the 

 one up Oil Creek, occurs, but very narrow. Opposite station 10 this bay 

 forms an oblong arm, crossing the Arkansas. Throughout the Creta- 

 ceous strata are conformable with those underlying, so far as could be 

 determined. The beds show very little variety, and but few fossils 

 were found in them, Inoceramus and Ostrea being among the most 

 numerous. At Canyon City a spring rises in the Lower Cretaceous beds 

 that attracts considerable attention on account of its agreeable taste 

 and freshness,, in consequence of a liberal supply of carbonic-acid gas. 

 This spring is located about a third of a mile west of Canyon. A quali- 

 tative analysis is given by Mr. P. Eraser, (Report United States Geolog- 

 ical Survey, 1869, p. 218,) showing the following constituents : 



NaO CO2 

 MgO CO2 

 CaO CO2 

 AI2 O3 

 Fe, trace. 

 I, trace. 



A comparison of the sections given on the Plate will show the uniform- 

 ity existing in the Cretaceous strata sloping off from the Front range 

 Above the yellow to brown sandstones follow series of white shales and 

 limestones, forming a row of hog-backs north of Canyon City, filled with 

 fragments of Inoceramus. Westward, beyond station 10, Cretaceous Nos. 

 2 and 3 appear, with Ostrea complexa in light-gray to yellowish shales. 

 They have a gentle dip eastward, varying several times on account of 

 small local changes in the niveau of the valley. A sandstone that may 

 belong to No. 4 or 5 — no fossils were found in it — overlies some of the 

 limestones of that region. Apparently Cretaceotis Nos. 2 and 3 rest im- 

 mediately upon granite, and it seems doubtful whether any considerable 

 thickness of older strata would be found below. On the south side of 

 the Arkansas, No. 1 rests upon the granite, so that it is likely to extend 

 northward far enough to underlie the succeeding strata. 



Along the Front range it becomes a matter of interest to determine 

 the end of the Cretaceous and the beginning of the Tertiary. It seems 

 that the age of the Lignitic group following the Cretaceous can only be 

 settled by very careful and accurate observations. It is possible that 

 the coal-beds farther north and those of Colorado may not be the same 

 geologically; and while the determination of their exact position is at 

 present based upon paleoutological evidence, it does not seem to confirm 

 either their Cretaceous or Tertiary age. In section a we found no coal, 

 although some of the bluffs showed dark shales resembling those farther 

 south of Canyon, where the coal occurs. 



Some distance from the mountains a number of bluffs occurs along the 



