ALGONKIAN AND CAMBRIAN EXPOSURES. 257 



Cretaceous age, probably owes its position to a pre-Cambrian ridge which 

 acted as a buttress or point d'appui to the forces of compression which pro- 

 duced this most remarkable and exceptional anticlinal fold of 30,000 feet of 

 practically conformable beds. The series of schists, slates, and quartzitesof 

 the Black Hills, which have hitherto been classed as Archaean, are probably 

 of Algonkian age also. 



In the Rocky Mountain region Mr. Arnold Hague found a considerable 

 thickness of quartzites resting on the Archaean in the Medicine Bow range at its 

 northern extremity, and an isolated patch of quartzite and conglomerate is 

 known to exist on the east flanks of the Colorado range near Boulder. In 

 the hills east of the Arkansas river at Salida and south of the South park, Mr. 

 Whitman Cross discovered a thickness of about 10,000 feet of slates and schists 

 entirely distinct from the Archaean and probably unconformable with it. On 

 the north slope of the San Juan mountains near Ouray, I have found over 

 10,000 feet of closely folded quartzites, conglomerates, and slates of pre-Cam- 

 brian age, and believe that the Quartzite peaks in the southern portion of 

 this region are probably composed of the same series of rocks.* Quartzites 

 have also been noticed connected with the Archaean of the southern end of 

 the Sangre de Cristo range which may on general grounds be assumed to be 

 the remnants of some Algonkian beds. 



While these various exposures are too isolated and have been too little 

 studied as yet to justify an attempt at correlation between them, they are 

 easily distinguished from the Archaean or basement rocks even when not 

 found directly associated with them. The latter, so far as the great areas 

 exposed have been studied, are distinctly crystalline, consisting mainly of 

 granites, gneisses, mica and hornblende schists, with none of the limestone 

 or apparently fragmentary beds which confuse the student of Archaean de- 

 velopments in the east ; while in the former, secondary alteration is either 

 very slight throughout the series or limited to certain beds, so that there can 

 be no doubt of their clastic or mechanical origin. 



The character of the material of which they are composed and their great 

 thickness show that they result from a long-continued abrasion of high 

 Archaean laud-masses in their near vicinity. It is to be noted, moreover 

 that all the Algonkian exposures, with the exception of that near Salida, are 

 on the outer flanks of the area which has been designated the Rocky Mount- 

 ain region. Their beds are steeply upturned or sharply folded, and all 

 Cambrian or later sediments rest unconformably upon them, as upon the 

 Archaean ; hence there must have been at least two and possibly more oro- 

 graphic movements between Archaean and Cambrian times. 



Cambrian Exposures. — At the base of the Paheozoic section in the Wasatch 

 mountains, as exposed in Big Cottonwood canon, are 12,000 feet of quartzites 



* This opinion is confirmed by Mr. Van Hise, who has visited this region during the past summer. 



