REPORT OF THE DIRECTOR 1920-21 89 



in the schistosity and banding from a few degrees east of north 

 in the southwest part of the quadrangle to an east-west direction 

 farther north, differences of 90 ° being observed even within a mile; 

 and the Algonkian sedimentaries change their strike from north- 

 west in the southeast corner to nearly east-west in the northwest 

 part of the sheet. On the Silverton quadrangle, however, which is 

 directly south of the Needle Mountains quadrangle, Whitman Cross 

 (1905) and his associates found that the Archean gneisses and schists 

 show in their schistosity and banding a distinct northeast-southwest 

 strike and a nearly vertical attitude. The Proterozoic (" Uncom- 

 pahgre ") quartzite is too massive to make out its structure, but 

 it is in that case also clear that the structure of the broad anticlines 

 of late age is independent of the Algonkian structure. On the 

 Engineer Mountain quadrangle (Whitman Cross & A. D. Hole, 19 10) 

 which is southwest of the Silverton quadrangle and between the 

 Needle and the Rico mountains, the Archean line of schistosity swings 

 from a north-south direction in the north, into a northeast-south- 

 west and finally into an east-west direction, while the Uncom- 

 pahgre, exposed in the northeast is thrown into compressed folds, 

 whose boundaries run west-northwest. 



On the Rico quadrangle, which is in the southwest corner of 

 Colorado, bordering the St Juan mountains, Whitman Cross and 

 Ransome (1905) found the Uncompahgre formation, consisting 

 mostly of quartzite, likewise striking in a northeast direction (N io° 

 — 30 E), and steeply dipping toward the southeast. Other quad- 

 rangles as the Pueblo, southeast of Pike's Peak, where Gilbert found a 

 north to northwest direction of the schistosity in the Precambrian 

 rocks, support the conclusion that the Precambrian rocks where ap- 

 parently least disturbed by later folding show a north-south to north- 

 east-southwest direction in their original structure. This original direc- 

 tion has been greatly disturbed locally by the northwest direction of 

 the mountain system imposed upon it. 



The bending of the Precambrian rocks in the Needle Mountains 

 quadrangle seems to be an expression on a smaller scale of this 

 general tendency of all Precambrian Rocky mountains axes to turn 

 toward the northwest, for as Suess states (v. 1, p. 726) all the Ar- 

 chean masses of the Rocky mountains strike from south to north 

 with a tendency to turn toward north-northwest or northwest. 



The most important area for the proper conception of the Pre- 

 cambrian fold system of the Rocky mountain region is, however, 

 the Yellowstone Park and the country adjoining it north and east; 

 or in other words north Wyoming and south Montana. Here, 



