78 GEOLOGICAL SURVEY OF THE TEERITORIES. 



found, still lower down the river, in a bed of blue limestone, an Orhicu- 

 la, plainly a Carboniferous specimen. 



The evidence seems clear that all the rocks on Eagle Eiver are of 

 Carboniferous age. As we continue down the Eagle Eiver to the north- 

 west, toward its junction with the Grand, there is a very curious twist 

 in the bed, which can hardly be described except by an illustration. 



Hitherto the great mass of the strata has been inclining from the 

 range of the Holy Cross, but here w€ seem to have met the force which 

 acted from the direction of the Blue Eiver range, by which the dip is 

 suddenly changed by a remarkable cause from a northeast to a north- 

 west direction, and while some portions of the group incline at an angle 

 of not more than 20^, others stand nearly vertical, and in some in- 

 stances have passed a vertical. We have here exposed in Eagle Eiver 

 a series of curiously-variegated beds, at least 4,000 feet in thickness, 

 of Carboniferous age ; above them 1,200 to 1,500 feet of brick-red sand- 

 stones and clays, probably Triassic, and above them 200 feet or more 

 of Jurassic age ; then overlaid by a heavy bed of quartzite, 150 feet 

 thick, undoubtedly No. 1, or Dakota group. Above the last are the 

 black plastic clays of No. 2, in a nearly horizontal position. Overlying 

 the clays is a bed of dark limestone filled with fossils, as Inoceramus, 

 Ostrea congesta, and other forms of mollusks, with abundant iish-remains, 

 but so broken that scarcely a good specimen could be found. 



At this point we left Eagle Eiver and struck across the country north- 

 northeast through a synclinal depression, or a sort of basin, from five to 

 eight miles in diameter, with the quartzites of the Dakota group rising 

 up with a gentle slope on all sides, while the high divide on the north 

 side of Eagle Eiver is elevated by the dikes of basalt so as to form a 

 rim connecting the east and west sides of the basin. Eagle Eiver, be- 

 low the point where we left it, just before it joins the main stream, cuts 

 through l^o: 1 and a great thickness of the brick-red beds, which are 

 elevated so as to show a general dip east and northeast, while on the 

 opposite side of the synclinal the dip is south and southwest. 



The quartzite bed of the Dakota group gives character to the slopes 

 as well as to the topography of this immediate region. The strata in- 

 clining to the north and northeast has been elevated by the Holy Cross 

 range, while the almost vertical beds on the opposite side of the syn- 

 clinal have been lifted to their present chaotic iDOsition by two forces ; 

 one acting from the Blue Eiver range, and the other arising from the 

 outfl^ow of igneous matter in the form of dikes. We may state, in gen- 

 eral terms, that in the Middle Park and its surroundings the complica- 

 tions in the positions of tlie strata of the various formations have been 

 produced by igneous eruptions. The terraces along Eagle Eiver ought 

 to receive a passing notice. There are usually two of them. Varying 

 from 10 feet to 50 or 100 feet above the river, usually covered with 

 rounded bowlders, there is a good thickness of the drift on them, bdt 

 the underlying strata of the original formations are everywhere exposed, 

 showing that the valley is for the most part one of erosion. 



Not unfrequently the river cuts a narrow channel across the upturned 

 edges of the basis rocks, which, in this way, may be traced across the 

 valley from bluff to bluff. The height of the divide overlooking Eagle 

 Eiver is 1,450 feet above the river-bed, and this may be taken as the 

 aggregate thickness of the entire Cretaceous group in this basin. All 

 the divisions appear to be represented ; and what we usually regarded 

 as No. 5 I estimated as 400 to 500 feet thick, composed of brown arena- 

 ceous shales, with irregular beds of sandstone, varying from a few feet 



