FORMATIONS OF THE FRONT RANGE 429 



are a few feet of transition beds consisting of an alternation of shales with 

 thin bedded, brown sandstones. The lower sandstone series is thicker 

 than the upper, often somewhat softer, and it contains shale partings at 

 some localities. The middle shale member appears to be present 

 throughout, but generally it is covered by talus. In the region west of 

 Denver the " Dakota' 1 formation is described by Eldridge as having a 

 thickness of 225 feet, consisting of two or three nearly equal benches of 

 massive sandstone separated by thin bodies of clay, a characteristic con- 

 glomerate at the base and a zone of hard, white, slaty shales 10 to 30 feet 

 thick at the top, transitional to the Benton. Fossil plants, mainly leaves 

 of the deciduous trees, and enormous fucoids are stated to occur from base 

 to summit. The sandstones vary from cross-bedded toslabby and often 

 some layers are ripple marked. The basal conglomerate varies from 

 almost nothing to 30 feet in thickness and is composed of well rounded 

 pebbles from the size of a pea to a diameter of 1 inch. 



In the Denver region there are generally two beds of clay — one mid- 

 way in the formation and the other nearer the summit. The thickness 

 of these varies from 2 to 8 feet, and the material is in part a typical fire- 

 clay of blue or blue gray color, fine, even grained, and very compact; 

 but in places it has intercalated sandy shales. The " Dakota " forma- 

 tion thins out and disappears for several miles in the vicinity of Golden. 



Doctor Peale* gives the following section of " Dakota " sandstone at 

 the mouth of the South Platte canyon, Colorado : 



Feet 



Gray and yellow sandstones 70 



Shaly sandstones, fossiliferous 12 



Fine grained white sandstones 3 



Rusty yellow sandstone . 245 



The total thickness of these members is 330 feet. The fossils referred 

 to were determined by Professor Lesquereux and included a Proteoides 

 very near P. acuta, H. At Perry (Pleasant) park Doctor Peale meas- 

 ured 213 feet of "Dakota" sandstones, but found its upper and lower 

 contacts obscured by talus. The tripartite succession is plainly shown in 

 figure 2, plate 33. The sandstone appears again for several miles in the 

 Garden of the Gods region, where Doctor Peale reports 257 feet of ex-r 

 posed beds, consisting of 200 feet of massive sandstones above, underlain 

 by a finer grained sandstone in part yellow, containing fragments of 

 Lingula and a lignitic layer with vegetal fragments. My own measure- 

 ment in vertical beds near the Gateway gave considerably less thick- 

 ness, and the formation was found to consist of two massive beds of 

 sandstone apparently with a thin series of shales between. The great 



*F. V. Hayden : U. S. Geoi. and Geog. Survey Terr., Seventh Ann. Rept. (for 1873), p. 195. 



