366 Sehuehert — Cambrian of the Grand Canyon of Arizona. 



The material is of the rocks immediately beneath and 

 the old regolith has had but little transportation and 

 assorting'. 



The character of the basal 35 feet of the Tapeats shows 

 little assorting and the thick beds are composed of decid- 

 edly cross-bedded tine-grained conglomerates with sub- 

 angular pebbles up to 10 mm. across. In these strata 

 are also interbedded short lenses up to 4 inches thick of 

 maroon colored shale. The material of these basal strata 

 is essentially the Vishnu, fragmented, with many pebbles 

 of red jasper (the red jasper probably came from some 

 nearby outcrop of Unkar), the whole being of a blotchy 

 maroon-yellowish color. Then comes 10 feet of conglom- 

 erate, maroon in color. Higher, the coarse sandstone 

 with layers of conglomerate is more evenly bedded in 

 beds from 1 to 2 feet thick, separated by irregular layers 

 of dark green shale. As one rises through the Tapeats it 

 is seen that the material becomes finer grained and is 

 better assorted, and that the shale zones become more 

 and more prominent, though the muds do not dominate 

 the Tapeats member until near the top and then the 

 whole changes over into a deep blue shale that weathers 

 an olive-green. These data show that the greater part 

 of the Tapeats is a delta deposit on the land side rather 

 than on that under the influence of the invading sea. 



The Tapeats sandstone passes gradually into the 

 Bright Angel formation of fine sandy soft shales with 

 interbedded shaly sandstones, here 332 feet thick accord- 

 ing to Noble's measurements (fig. 4). Everywhere it 

 makes gentle slopes and little cliffs where the sandstones 

 are. On the weathered surface it is in general olive- 

 green in color, but on fresh fracture is a bluish micaceous 

 (muscovite) shale, interbedded with many thin and thick 

 beds of dark or brownish cross-bedded sandstones, many 

 of which are highly giauconitic. When the sandstones 

 have obolid shells, then they become more or less calca- 

 reous or even arenaceous giauconitic limestones. These 

 sediments show clearly that the sea was not deep, but 

 that it probably covered wide flats to a depth of less than 

 150 feet, not many tens of miles from the shore. Deposi- 

 tion was fairly rapid. When the waters were fairly 

 clear and were depositing sandstones, glauconite was 

 forming and there were incursions of inarticulate brachi- 



