102 ANNALS NEW YORK ACADEMY OF SCIENCES 



Origin and Nature of the AlgonMan Sediments 



The nature of the Algonkian rocks has already been partly discussed 

 and a very general description given in the sections by the Fortieth Paral- 

 lel geologists and Dr. Walcott. These may be briefly summarized as fol- 

 lows: The prevailing rocks are quartzites and interbedded shales, the 

 quartzites being mostly light colored, white and yellow to light brown; 

 the shales, dark purple and green to black. Toward the top of the series, 

 thin sheets of conglomerate occur in the quartzites, in which small well- 

 rounded pebbles of quartz and quartzite are abundant. In a dark shale at 

 the base of the series, mud cracks are abundant. To these characters may 

 be added several others observed by the writer and also mentioned by 

 Blackwelder. The quartzites are often prominently cross-bedded, the dis- 

 cordant angularity of the beds being usually of small amplitude. Eipple 

 marks of the long parallel type are often common in the sandy shales. 

 Limestones are totally absent, and though conditions favorable to the 

 preservation of fossils seem to be abundant and right, no organic remains 

 have been discovered. 



From the intermediate geographical location of the Big Cottonwood 

 section with respect to the Grand Canyon section on the south and the 

 many occurrences of thick pre-Cambrian sediments to the north in Idaho 

 and Montana, and from the fact that many of the above mentioned fea- 

 tures of the "Wasatch Algonkian have also been recorded from these other 

 localities, it seems logical to suppose that they must have the same or a 

 very similar origin. Upon whatever basis one is explained, the rest will 

 probably also be explicable. Limestones and dolomites are met with in 

 the northern and southern series and show that those regions had more 

 varied conditions of sedimentation, involving periodic inundations of the 

 sea, unless they are of fresh water origin. The major portions of the 

 rocks are, however, clastic sediments and show physical characteristics 

 which point to a continental origin. Shrinkage cracks and ripple marks 

 in the shales and shaly sandstones indicate extensive mud flats comparable 

 to the flood plains of many of our large rivers. Cross-bedding of the type 

 here found suggests shifting water currents such as those of terrestrial 

 rivers rather than wind. So that if we postulate a river origin for most 

 of the quartzites and shales, we have at once a complete explanation of 

 the physical characters already noted and the conspicuous dearth of fos- 

 sils. BarrelP^ argues for the dominant flood plain origin of mud cracked 



^3 J. Barrell : "GeoloRical Importance of Sedimentation," Jour, of Geo!.. Vol. XIV, pp. 

 553-568. 1906. 



