160 GEOLOGY OF THE DENVER BASIN. 



what is probablv in reality a very sharp line, lies in the soft and easily 

 destructible character of the clays and friable sandstones constituting the 

 strata of both formations near their contact. Even along the western 

 border, where the edg'es of the strata are upturned, it is difficult to find 

 good outcrops at this horizon. 



The lowest beds seen which have been referred to the Denver scries 

 are sandy layers consisting of a mixture of quartz and feldspar plainly 

 derived from Archean rocks, with other minerals, such as augite, horn- 

 blende, biotite, and feldspar, which exhibit properties belonging to the 

 constituents of eruptive rocks. Except, however, occasional pebbles of 

 distinct andesite, which sometimes occur in these layers, there may be 

 nothing to suggest the character of these mineral particles until they are 

 subjected to microscopical examination. It is then found that at least a 

 portion of the feldspar contains ore grains or microlites of zircon, apatite, 

 and augite, with more or less typical glass inclusions, all these indicating 

 an eruptive origin. As to the hornblende and biotite, it is not always 

 possible to determine their source so clearly, but the augite is, like the 

 feldspar, plainly to be considered as a former component of some eruptive 

 rock. 



Although much of the lower part of the Denver series is almost if not 

 entirely free from Archean debris, the actual base contains a mixture of 

 materials. A consideration of the facts leads to the belief that all mineral 

 particles of noneruptive origin contained in the lower layers of the series 

 belong to the movable sands of the sea-bottom upon which the new forma- 

 tion began. The base of the Denver series is thus determined by the first 

 appearance of eruptive material among the particles derived from the crys- 

 talline or older sedimentary rocks. This line could be determined with 

 accuracy were there sufficient outcrops along its course. 



Strata of Section Ravine. TllC single CXJIOSIU'C along the WCSteill lillC which 



shows clearly and sharply the transition from the beds of the Arapahoe to 

 those of the Denver formation is that in Section Ravine, upon the south- 

 western slope of Green Mountain. The Arapahoe formation ends with a 

 series of clays, which are succeeded at a certain point by a hard, coarse- 

 grained sandstone or grit layer, in which quartz grains are very prominent, 



