TRIASSIC. 533 



K-L 13. BLACK HILLS, SOUTH DAKOTA. 



The Spearfish formation, the uppermost division of the red beds in the Black 

 Hills, was described by Darton ^^ as probably Triassic. He states: 



The formation consists of from 350 to 500 feet of red sandy clays, with intercalated beds 

 of gypsum which sometimes are 30 feet thick. * * * It is thought to be Triassic in age 

 because it lies unconformably beneath marine Jurassic deposits and is underlain by the Minne- 

 kahta limestone, which is known to be of Permian age. 



The tendency is now to correlate the Spearfish formation with the upper part 

 of the Chugwater formation, which is also regarded as Triassic, the lower part of the 

 Chugwater probably belonging to the Permian. 



L 11. NORTHEASTERN OREGON AND WESTERN IDAHO. 



Lindgren ^^ discusses briefly the occurrence of the Triassic in western North 

 America and describes the Triassic strata in eastern Oregon and adjacent Idaho as 

 follows : 



With widespread Triassic areas in California, Idaho, and Nevada, and with similar areas 

 north of the international boundary line, it seemed strange that no rocks of the same age were 

 found in Oregon or in Washington. This gap has been partly bridged by the discovery of very 

 extensive Triassic beds in the Eagle Creek Range, extending from there across Snake River into 

 Idaho, to the vicinity of the Seven Devils. If the sedimentary rocks in the vicinity of Hunting- 

 ton belong to the same age, as is possible though by no means certain, the area of the Triassic 

 would be stiU further extended. 



A characteristic feature of the Triassic of the Eagle Creek Range is the occurrence of large 

 masses of limestone and some shale, with an abundance of more or less altered lavas poured out 

 during the time when these beds were being deposited. 



The Triassic sediments are best exposed on Eagle Creek. All along the foothills of the 

 lower Powder River the Triassic lavas contain smaller bodies of limestone and shale, but on 

 Eagle Creek below the forks, the sedimentary series prevails and consists of calcareous shales 

 and limestone ia horizontal or slightly inclined position. Volcanic breccias are interbedded with 

 the limestone. The total thickness of the series, including the volcanic beds, is probably 

 several thousand feet. Many of the exposed limestone masses are several hundred feet in thick- 

 ness. Above the junction * * * tj^g limestones become converted into marbles and the 

 volcanic breccias into schists, whUe the whole series acquires a dip of 60° E. 



Dr. T. W. Stanton examined fossils collected one-third mile below the mouth of East Eagle 

 Creek and reports them to consist of numerous specimens of Halobia and two indeterminable 

 fragments of an ammonite. The Halobia is apparently an undescribed species, but the genus 

 itself is characteristic of the Trias. Another lot, collected from the limestone bluff on East 

 Eagle Creek 2 J mUes above its mouth, contains Pentacrinus columns with spines and fragments 

 of tests of echinoids. From the Miles placers, 1^ mUes below the mouth of East Eagle Creek, I 

 obtained through Mr. F. R. Mellis, of Baker City, a cast of a gigantic gastropod found in the 

 limestone bedrock during drifting operations on this claim. Dr. Stanton remarks that it has the 

 form of a very large TurriteUa or Pseudomelania. Nothing of this character approaching it 

 in size has been described from the west coast, but similar forms described as Chemnitzia and 

 Pseudomelania are known from the Trias and Jura of Europe. The total length of this cast is 

 8 inches and its diameter at the thickest end 3 inches. 



Along Snake River canyon, below the mouth of Pine Creek, Triassic lavas and tuffs are 

 again exposed in the bluff below the Neocene basalt flows which cap the hiUs. These Triassic 

 igneous rocks contain thin beds of black shales and limestone with imprints of a large species 

 of Daonella or Halobia. A limestone mass 4 mUes below Ballard Ferry contained a Lima with 



