214 GEOLOGY OF THE DENVER BASIN. 



of the various formations in question. 1 Little can at present be added to 

 that summary, which is much more complete in details than that which is 

 to follow. 



The Middle Park beds. — Across the Colorado Range from Denver is the 

 elevated basin of Middle Park, containing- a great series of strata repre- 

 sented upon the Ilavden map of Colorado as Laramie. These beds were 

 of special interest at the time of their discovery by Marvine, in 1873, by 

 reason of the marked angular unconformity with the underlying Cretaceous 

 section, which was taken as confirmatory evidence that the "Lignitie" beds 

 were Eocene. When the Laramie was defined as conformable with the 

 Cretaceous series below, the facts of the Middle Park beds were conven- 

 iently regarded as of local importance, a procedure commonly resorted to 

 when new discoveries controvert old ideas. 



As Marvine's description 2 of the Middle Park beds showed them to be 

 not only unconformable with the Cretaceous section, but also lithologically 

 similar to the Denver beds, a correlation with the latter was at once sug- 

 gested, and the region was visited in 18<S'.i by Mr. G. L. Cannon, jr., of 

 Denver, and in 1891 by the writer, both under direction of Mr. Emmons 

 The result of these visits was published in 1892. 3 



The strata in Middle Park designated as Laramie by the Hayden 

 survey extend from a point south of the Grand River northward, forming 

 the high divide between Middle and North Parks, and occupying a large 

 area in the latter region. According to Marvine these strata are . r >,f><)0 

 feet in thickness. And Marvine did not include with them an underlying 

 bedded formation, called by him "doleritic breccia," to which he assigned 

 a maximum thickness of 800 or 900 feet. In the article above cited the 

 writer has reviewed in detail the facts and descriptions of Marvine, and 

 in the following paragraphs will be given the facts concerning these strata 

 as now understood. 



The "doleritic breccia" of Marvine is a series of dark tuffs, conglom- 

 erates, and breccia beds, made up of a large series of andesitic fragments, 



1 Post-Laramie deposits of Colorado: Am. Jour. Sci., 3d series, Vol. XLIV, 1892, pp. 19-42. 

 ^Seventh Ann. Rept. U. S. o. and G. S.,ibr 1873. 



3 The post-Laramie beds of Middle Park, Colo., by Whitman Cross: ProG:Colo. Sci. Soc.,Vol. 

 Ill, 1891. 



