THE 



JOURNAL OF GEOLOGY 



SEPTEMBER-OCTOBER, 1908 



THE RED BEDS OF NORTHERN COLORADO^ 



JUNIUS HENDERSON 

 University of Colorado, Boulder, Colo. 



The perennial problem of the age of the so-called Red Beds of 

 Colorado received some attention from the State Geological Survey 

 during the field-season of 1907, and owin^ to the uncertainty as to 

 when the results may be pubhshed by the survey, it is deemed best at 

 this time to give to the pubhc a brief statement of the important facts. 

 These beds were designated Jura-Trias by the early surveys, chiefly 

 on account of lithologic resemblances to formations found elsewhere. 

 Since it became evident that at least the lower portion in many places 

 is much earlier, the term Red Beds has come into general use. Qur 

 work on these formations in 1907 was confined to the foothill region 

 north of Boulder. At the northern boundary of Colorado we found 

 a chert concretion zone containing Spirijer centronatus Winchell, 

 Cranaena subelliptica var. hardingensis Girty, and Spirifenna solidi- 

 rostris White, a fauna which is considered Mississippian and is found- 

 on the ea^t side of the Front Range at Canyon City and elsewhere. 

 The first-named species and others of like age were long ago reported 

 from that region, but the record had little value because the position 

 in the formation was not given. We traced the chert zone for eight 

 or nine miles southward, finding it everywhere within a few feet of 

 the contact of the conglomerate with the granite and uniformly con- 

 taining fossils, but found none farther south. The coarse sandstone 

 and conglomerate series in which this fossiliferous zone occurs appears 



I By permission of the State Geologist of Colorado. 

 Vol. XVI, No. 6 491 



