140 C.W.Hayes — Expedition through the Yukon District. 



glomerate and schist. The district between the Donjek and 

 Koidern rivers is composed almost entirely of white marble and 

 talcose schist, and is the largest observed area of sedimentary 

 rocks between the St Elias mountains and the Yukon. 



Rocks of Scolai Pass. — As already described, two slightly diver- 

 gent ranges, separated by the Chittenah valley, extend toward 

 the west and northwest from mount St Elias. The geology of 

 the northern range is simple. In the walls of Scolai pass, by 

 which the range was crossed, its stratigraphy and structure are 

 magnificently displayed. The rocks are comparatively recent, 

 for the most part Carboniferous, Triassic, and Cretaceous. A 

 bed of limestone about 500 feet thick contains many crinoids 

 and corals, probably of Carboniferous age. Above it are red 

 sandstone and jasper and a'great thickness of black shale. Col- 

 lections of fossils from the limestone and the black shale were 

 made, but before reaching the coast they unfortunately were 

 lost, with the exception of a single small piece of shale ; this, 

 however, contained several tolerably perfect impressions and 

 was submitted to Professor Alpheus Hyatt for identification. 

 He says : " The fossils in the shale are clearly the remains of a 

 Monotis of a Triassic type, allied to M. suhcircularis, Gabb, a char- 

 acteristic Triassic form in California. This one seems to be 

 distinct specifically, but is evidently of the same age.'' 



Interbedded with these sedimentary rocks and penetrating 

 them as dikes are fine-grained, greenish amygdaloid lavas form- 

 ing perhaps half of the whole rock-mass. The structure of the 

 range consists essentially of a broad, gentle synclinal, with a 

 highly contorted belt on either side. 



Excellent examples of typical fan structure were seen in 

 the intensely plicated rocks which form the abrupt northern face 

 of the range. This structure is remarkably well shown in the 

 sides of the gorge from which Kletsan creek issues. The 500- 

 foot stratum of white limestone above referred to is folded in 

 with dark greenish-black eruptive rocks so as to form a double 

 V ; the overturned southern synclinal limbs dip southward about 

 30° and 45°, while the normal northern limbs are nearly hori- 

 zontal. 



This plicated belt on the northern side of the mountains is 

 about six miles wide, and south of it the synclinal in which the 

 beds are practically horizontal (coinciding with the axis of the 

 range) occupies a belt from twenty-five to thirty miles in width. 



