Sclmchert — Cambrian of the Grand Gavyon of Arizona. 369 



die third of the Muav many of the shaly limestones are 

 intraformational conglomerates with the pebbles small, 

 flat, and more or less rounded on the edges. It is an 

 interesting shallow-water, near-shore, marine deposit. 

 This is shown in the great abundance of annelid burrows, 

 in the intraformational conglomerates, and in the vari- 

 able nature of the calcareous deposits. 



As to whether there is a break in sedimentation 

 between the Muav and Bright Angel formations, or, in 

 other words, between the Middle and Upper Cambrian, 

 the writer could not determine in his limited time. In 

 any event, one has no difficulty in the field in drawing the 

 line of separation between the blue-green shales at the 

 top of the Bright Angel formation and the thinly lam- 

 inated calcareous shales and impure limestones at the 

 base of the Muav. Doctor Noble in a letter to the writer 

 states that he had even less difficulty in laying the line of 

 separation between these formations to the west in the 

 Shinumo quadrangle, for here the change in lithology is 

 sharper than about El Tovar and to the eastward. 



Fossils of the Muav .— While recognizable fossils are 

 extremely rare in the Muav formation, yet annelids were 

 exceedingly common. The writer has not seen a Paleo- 

 zoic marine deposit more bored into and consumed by 

 mud eaters than this one, and these burrows are most 

 prevalent in the calcareous zones. Worm castings are 

 often well preserved in the thin-bedded shale zones, and 

 occur as little confused heaps or in circular sausage- 

 like strings. 



At about 200 feet above the base of the Muav Noble 

 collected and Kirk identified the brachiopod genus Syn- 

 trophia and, through fragments, the following trilobite 

 genera: Finkelnburgia, Pagodia, Anomocarella and 

 Ptychoparia. About 50 feet lower the writer found 

 pygidia of Saukia and cf. Neolenus. Near the top were 

 found by Walcott two large orthids of Jamesella f and 

 Billingsella. These fossils, and more especially the trilo- 

 bites, show that the Muav is of Upper Cambrian age. 



Am. Jour. Sci.— Fourth Series, Vol. XLV, No. 269.— May, 1918. 

 26 



