THE CANTWELL FORMATION 12 1 



rocks no longer remain massive, but show greater and greater 

 schistosity, the finer members passing finally into slates and 

 phylKtes, the coarser ones into mica schists. 



The massive conglomerate was examined with especial care 

 just east of the mouth of Jack River, where it forms prominent 

 cliffs along the southern face of the Alaska Range. The rock is 

 composed of grains, pebbles, and cobbles (observed up to 7 inches 

 in length) of quartz, quartzite, slate, rhyolite porphyry (?), and 

 perhaps other rocks, set in siliceous cement. Intercalated between 

 these massive beds are thinner members of sandstone, quartzite, 

 and dark-blue carbonaceous argillite. The formation is separated 

 here from the Paleozoic limestone along its southern border by a 

 narrow intrusion of granite, connecting eastward with a large area 

 that intercepts the sediments for a course of several miles. 



East of the granite intrusion the Cantwell formation reappears 

 in a more accessible portion of the Alaska Range embracing the 

 headwater region of Wells Creek. This area extends eastward 

 for 13 miles almost to the Nenana Glacier, where it is interrupted 

 by another great granite intrusion. The sediments show the 

 effects of more compression than do the massive beds to the west, 

 and in consequence display considerable schistosity. This Wells 

 Creek area was carefully studied in its western, central, and east- 

 ern portions. 



In its western portion, a good section was obtained up a small 

 draw from the valley gravels to the mountain top. The lowermost 

 1,500 feet consists predominantly of massive to somewhat schis- 

 tose graywackes of dark-gray color, composed of subangular to 

 rounded pieces of quartz and black slate set in siliceous cement. 

 This rock carries intercalated beds of black carbonaceous shales 

 with abundant impressions of leaves and of siliceous conglomerate, 

 in part somewhat schistose. The remaining 600 feet to the moun- 

 tain top is composed of prominent, heavy beds of a severely mashed 

 phase of the conglomerate, light greenish to yellowish in color, in 

 places so schistose that the original texture is nearly obliterated. 

 The average rock, however, shows the coarser components as elon- 

 gated lenses, resembling so-called "stretched-pebbles," enwrapped 

 by the finer constituents, which have been rendered in part 



