T 



nam.] SECTION IX CONGRESS CANYON. 479 



boniferous form of P. semiretieulatus, which Dr. E. SeheHwien* 1ms 

 named var. bathyoolpo8; but in P. Ivetit the ribs of the shell and the 

 spines are more strongly developed and the medial septum in the 

 smaller valve is divided. P. Ivesii should be considered a variety of J\ 

 semircticulatus and not a distinct species. 



The principal terraces are developed at the horizons of the Upper 

 Tonto (IV), and at the base and top of the Lower Aubrey sandstone 

 (Via). They are indicated by the reduced slope of tin; cliff in the 

 accompanying- section ( Fig.32). The actual slope of these terraces could 

 not be given for want of space. 



The principal cliffs are formed by the Archean, the (hand Canyon 

 and Lower Tonto series and the Red Wall. The picturesque carved (or 

 incised) forms of the latter have a remarkable resemblance to the 

 Dolomite cliffs of the Upper Trias in southern Tyrol. 



The following table gives the results of a more detailed examination 

 of the Tonto series (III and I V) : 



III. 1. Coarse, red, cross-bedded sandstones, with pebbles of quartz at the base 



and Seolithus at the* top ca . 200 



2. White sandstone, spotted black 30 



IV. 'A. Thin-bedded, brown, qnartzitic sandstones and shales M 



■1. Yellow and chocolate-colored, sandy shales, alternating with cross- 

 bedded sandstones and conglomerates J5 



5. Well-defined bedof in-own sandstone with glanoonite(t), containing 



Obollchi mm , 



(i. Greenish or snuff-colored, shaly sandstone with worm tracks (Crugi- 

 ana), ripple-marks and glauconite ( f) (OboUelapoltiaH&H 1 Lingvla 

 montuula Walcott I ) 65 



7. Same rocks as in r> and <>. Large ripple-marks in the lower part, and 



in 6he upper part a glauconite layer 5 to IB cm. thick. ObollelaBj>.1.. 32 



8. Snuff-colored sandstone, forming a well-defined cliff 8 feet high. In 



the upper part some calcareous shale 85 



D. Greenish or snuff-colored shales with small ripple-marks, forming a 



gentle slope 85 



10. Same beds as in H. In the upper, a bed of limestone forming a small 



cliff 44 



it. Greenish shales, with small cliff in the midst formed by a greenish 



sandstone 85 



The section shown in Congress canyon is in many respects incom- 

 plete. The following table, made by 0. I). Walcott 96 at the Kaibab 

 Plateau on the north side of the Grand canyon, gives the complete 

 section (Roman numerals denote corresponding beds observed in Con- 

 gress canyon): 



Veot. 



Tertiary 815 



Cretaceous 3 ( 095 



•To whom I am indebted for the determination of the two Prodnctus. Compare 



the figures of /'. eemiretioulatm While, (J. and (i. Surveys \V. of 100th meridian, Vol. 

 iv. p. Ill, PI. vim, flg. 1, and Pro&tctua Sp. '. Meek. I'. S. (i. Explor. 40th Par. Vol. 

 iv p. 67, PL vii, tig. 6, 



