THE NORTH PARK. 127 



group of volcanic rocks, stretching in an east and west line, between two 

 great longitudinal ranges of highly-altered metamorphic schists and gneisses. 



On the west side of the Park, north of the basaltic area, the only 

 Mesozoic beds observed were Dakota sandstones and the Colorado group, 

 and these are traced with difficulty, owing to the rough nature of Archaean 

 foot-hills and great accumulation of glacial detritus, which not only con- 

 ceals the Cretaceous strata, but renders travelHng impracticable. The 

 Dakota beds, resting upon the Archaean, dip at angles varying from 25° to 

 50°. The Colorado clays incline much more gently, and stretch out into 

 the Park basin, with a dreary arid appearance, with scarcely any vege- 

 tation, and dotted over with alkaline incrustations, chiefly sulphates of 

 soda and magnesia, Avhicli permeate the argillaceous beds. Along the 

 Archaean spur, which projects out from the main range and terminates in 

 Crawley Butte, are found, on the north side, all the Mesozoic strata from 

 the Red Beds to the top of the Colorado group, inclined at low angles; 

 while, upon the south side, the horizontal Tertiary beds would appear 

 everywhere to abut against crystalline strata. 



Tertiary Beds. — Ovei-lying the uppermost Cretaceous strata repre- 

 sented within the Park, occur the beds of approximately horizontal Ter- 

 tiary deposits, to which allusion has already frequently been made. They 

 were rarely observed inclined at a higher angle than 4°. They lie uncon- 

 formably upon the older rocks, resting in places against every formation 

 from Archaean to the top of the Colorado group, and are seen in an undis- 

 turbed condition, resting against the basalts. They extend over the entire 

 Park basin, giving it the level, prairie-like aspect, which it presents from 

 all the higher elevations. Through these beds, the many streams of the 

 Platte drainage have worn their present channels, leaving everywhere long 

 bench-like ridges, with steep sides, which, although offering numerous good 

 exposures, appear in no case to have cut deeply into underlying strata, 

 making any determination of their thickness uncertain. Within the Park, 

 they probably do not exceed a few hundred feet. Lithologically, these 

 deposits possess a somewhat local character, the material of which the 

 uppermost beds are formed being derived exclusively from the relatively 

 narrow limits hemmed in by the Park walls, rendering any comparison 



