•28 W. Cross — Post Laramie Deposits of Colorado. 



In his report on Middle Park Marvine describes very clearly 

 the excellent section of the Cretaceous formations exposed in 

 a synclinal basin cat across by the Grand River for eight 

 miles above Hot Sulphur Springs. He also gives detailed 

 maps, and sketches from the master hand of W. H. Holmes. 

 He well understood the importance of the locality from the 

 standpoint afforded by his interpretation of the stratigraphy, 

 and since 1873 the "Laramie " of Middle Park has been cited 

 as probably the only decided instance of great unconformity 

 between this formation and the lower Cretaceous horizons. 



To summarize Marvine's description of the Grand River 

 section, he found the Cretaceous, from the Dakota to the Fox 

 Hills, inclusive, exposed on both sides of the syncline men- 

 tioned. Then comes a formation occupying the center of the 

 shallow syncline, which is described as follows :* " Above the 

 Cretaceous No. 5, the next youngest rock is a local occurrence 

 of volcanic doleritic material, consisting partially of subaque- 

 ous-arranged material — dolerite, tuff, and breccia — and partially 

 as accompanying lava-flows ; in all, reaching a maximum thick- 

 ness of 800 or 900 feet." This formation is thereafter usually 

 designated as "Doleritic breccia." "Resting upon the latter 

 [the breccia] when it occurs, but elsewhere upon Cretaceous 

 No. 5, and apparently conformable with the latter, except at 

 one point where there is a decided unconformability, is a series 

 of beds which reach a thickness of about 5500 feet." .... 

 " They are composed in part of sandy shales, in places more 

 or less argillaceous and quite soft, spaced rather regularly with 

 more prominent and characteristic horizons of coarse sand- 

 stones, which are often inclined to grits and fine conglomerates. 

 The texture of the latter is usually open and not firmly com- 

 pacted, while the material of which they are composed is 

 characteristically the debris of the Archaean rocks of the moun- 

 tains, granitic debris prevailing." . . . " Impressions of decid- 

 uous leaves are quite numerous at favorable localities and small 

 isolated patches, and one or two thin seams of carbonaceous 

 material were also observed. No other fossils were observed 

 in these beds. It has been strongly affected by the last great 

 folding accompanying the formation of the Rocky Mountains, 

 portions of it being abruptly upturned, together with the 

 underlying sedimentary rocks. In position and character, 

 therefore, this group of beds appears to be the equivalent of 

 the lignitic group east of the mountains." 



To the westward of this syncline, in which the formations 

 are apparently conformable, Marvine found that the " lignitic 

 beds " above the " breccia " in fact overlap the latter and rest 

 successively on the inclined strata of the entire Cretaceous 



* Seventh Ann. Rep. TJ. S. G. and G. S., pp. 156, 157. 1874. 



