516 Noble — Geology of the Grand Canyon, Arizona. 



a. Gray-green, pinkish-green, an$ brown micaceous shaly 



sandstones, cross-bedded and ripple-marked, varying 

 in character only through gradations in color. Fre- 

 quent arenaceous and argillaceous shaly partings occur, 

 causing the rock to weather like a soft sandy shale. 

 The shale partings are usually green. Some of the 

 sandy layers near the base show the " gnarled and 

 twisted structure." 1100' 



b. The green, gray, and brown beds of a pass upward 



through change of color into red and vermilion beds 

 of the same general character. These are likewise 

 micaceous shaly sandstones, cross-bedded and ripple- 

 marked, with arenaceous and argillaceous shaly part- 

 ings, which display well preserved mud-cracks. The 

 shaly partings are either green or red 1197' 



Total _ 2297' 



The summit of the Unkar section is obliterated in the beds 

 of division b by the plane of the pre-Tonto unconformity. 

 The highest beds of this division lie at the upper end of the 

 Fault Wash, dragged against the Vishnu schist on the north 

 by the great pre-Cambrian fault previously described, and the 

 whole overlain by the Tonto sandstone. 



In the uppermost part of division b are four thin sills of 

 rotten basaltic rock, weathering green and crumbling to small 

 fragments. They occur between the beds of vermilion sand- 

 stone and shale. Their intrusive character is shown by the 

 fact that they have baked and decolorized the vermilion 

 beds for a few inches both above and below the contacts, 

 changing the color to purple. The rock is too badly weathered 

 to allow a petrographic determination in thin section. 



In summary the fifth member of the Unkar may be charac- 

 terized as a series of micaceous shaly sandstones of uniform 

 character, varying only in color, and bearing marks of shallow 

 w r ater origin throughout. 



Comparison with the type section in Unkar Valley. — 

 A comparison of the above section of the Unkar group with 

 that described by Walcott (d) in the type locality 30 miles to 

 the east reveals the fact that the correspondence between the 

 tw T o sections is very close, both in thickness and in lithological 

 succession ; only in the lower portions do they differ materi- 

 ally. The type section in Unkar valley is characterized by a 

 greater thickness of the basal conglomerate, and by only a third 

 as much limestone in the members which correspond to the sec- 

 ond member on the Shinumo ; the deficiency in limestone is made 

 up by greater thickness of arenaceous and argillaceous shale. 

 Proceeding upward in the section there is a somewhat greater 



