DOMED CRETACEOUS STRATA. 447 



Iii the southern half the strata have a general inward dip at the outer 

 edge of the range, both in the spurs and adjacent benches; so that gentle 

 easterly dips arc found on the western side, northerly dips on the south, 

 and westerly dips on the east. In the interior this basin structure is in- 

 terrupted by dome-shaped uplifts, of which the most marked is that con- 

 nected with the central dioritic stock, from winch the stratified rocks dip 

 away with gently decreasing dips. This dome structure is sometimes 

 repeated on a smaller scale in the outlying buttes. An example exists 

 on the northeastern border between Little Elk and Big Elk creeks;* the 

 shales and sandstones dip about 30° in three directions from the center 

 of the dome, which has been eroded 300 feet lower than the sides, thus 

 forming a roughly circular basin a mile or two wide surrounded by lines 

 of cliffs. One small intrusive sheet can he seen in the upper strata, which 

 rapidly thins out. Still farther outward from the center of the dome the 

 strata have steep dips and contain numerous intrusive sheets or bedded 

 dikes. The eroded center seems to be due to the lack of protecting erup- 

 tive sheets at that point, making it easy for the erosive agents to cut deep 

 into the soft shales and sandstones. 



In the northern half of the mountains the dome structure is developed 

 with less regularity and a tendency to longitudinal uplifts with steeper 

 dips and sharp crumples, producing long-crested ridges. An interesting 

 case is found on the northern side of the deep transverse valley at the 

 head of Shields river, consisting in the southern end of a long anticlinal 

 dome, the strata dipping southward, eastward and westward within the 

 space of a mile. They are here interleaved with numerous sheets of in- 

 trusive rocks, which curve around the sides of the dome with them and 

 even preserve this parallelism in sharp minor crinkles a few hundred 

 feet wide, by which the lines of outcrop make elbows. The present cresl 

 of the dome is formed by a master sheet or laccolite sixty feet thick, 

 which dips off from three sides; but erosion has cut through it on the 

 axis of the dome to the underlying soft shales, exposing to view a trans- 

 verse dike of the same rock, apparently a feeder of the laccolite. The 

 close conformity in greater and lesser crumplings between the intrusive 

 and sedimentary rocks makes it necessary to suppose tli.it the elevation 

 took place after the intrusion of the former, for it docs not seem possible 

 that an intrusive rock could force its way into all the details of a sharply 

 crumpled surface. This being the case, the eruptive rocks were exam- 

 ined with considerable interest at one of the sharp twists for signs of 

 crushing, and with the expectation of some trace of the dynamic meta- 

 morphism so common in folded intrusive sheets of the Archean and 



I he topographic map i- nol reproduced here. 



