EASTERN FOOT-HILLS. 33 



Productus semireticulatus. 

 Productus cora. 

 Productus Pratteniamis. 

 Atliyris suUilita. 



Of these forms, P. semi-reticulatus would appear to be the most abundant. 



In the palseontological report of Mr. G. B. Grinnell/ accompanying 

 the report of Colonel Ludlow, on a " Reconnaissance of the Black Hills of 

 Dakota", he mentions finding, in one or two localities, Spirifer centronatus, 

 Winchell, but does not state in what part of the limestone series they occur. 

 This species, along the belt of the Fortieth Parallel Survey, is a character- 

 istic fossil of the Waverly group in the Wahsatch limestone, occurring in 

 several localities in the Wahsatch and Oquirrh Ranges of Utah. Its pres- 

 ence along the east base of the mountains, where heretofore only Primor- 

 dial and Coal-Measure species have been recognized, is of special interest. 



Triassic foemation. — Immediately overlying the Coal-Measure lime- 

 stone occur the heavy sandstone beds that have been referred to the Triassic 

 age. They are found all along the foot-hills of the range, the continuity of 

 outcrop being broken iiTi only a few places, and, in most cases, simply by 

 being concealed below the unconformable Tertiary beds. 



They are characterized by a prevailing brilliant red color, which, how- 

 ever, shades off into yellowish and whitish tints, and, near the top and bot- 

 tom of the series, show frequently reddish-gray bands. The deep brick-red 

 color, however, is so persistent as to form one of the most clearly-defined 

 geological horizons of the uplifted sedimentary beds. 



In thickness, the Triassic beds vary very considerably; thinning out 

 from 800 feet, as given in the general section, down to less than 300 feet. 

 They reach their greatest development to the southward in Colorado, be- 

 tween the Big Thompson and Cache la Poudre, while north of the railroad, 

 they appear much thinner, and, between Lodge Pole and Horse Creeks, 

 reach their minimum. Still farther to the northward, in the region of the 

 Chugwater, they again thicken, but scarcely attain the width of the beds 

 in Colorado. 



'Reconnaissance of the Black' Hills of Dakota, 1875, 76 and 77. 



3 D O 



