2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



calcareous content. Cross-bedding, sun cracks, ripple marks and the 

 Other usual Bcltian features characterize the series, but contemporane- 

 ous igneous rocks are evidently lacking, w^hich is also the case in the 

 Beltian strata of nearby western Montana. 



MIDDLE CAMBRIAN 



The Middle Cambrian series of the Pend Oreille region begins with 

 a quartzite. which is followed in turn by argillaceous shale and calcare- 

 ous formations. No mention is made of younger strata, and the pub- 

 lished structure sections indicate that the Cambrian is not overlain 

 by other beds. 



Gold Creek quartzite. — The Gold Creek quartzite, which is estimated 

 to average 400 feet in thickness, is easily distinguished from the 

 Beltian by its coarser grain. Some of the conglomeratic beds contain 

 pebbles up to 3 inches in diameter and cross-bedding is a characteristic 

 feature. Outcrops are conspicuous because of the resistance of the 

 rock to weathering. Unfortunately, the contact of the Gold Creek with 

 the Beltian rocks is not clearly exposed so that essential history is 

 lacking. 



Rennie shale. — This formation is only 50 to 75 feet thick and con- 

 sists of soft olive argillaceous shale, sometimes micaceous. Because 

 it is so easily eroded, the Rennie shale seldom crops out. In fact, the 

 fossils here described were collected from the bed of a brook. 



The published summary fails to mention the fossiliferous limestone 

 nodules present in the Rennie shale. They evidently are about the same 

 size and shape as similar nodules obtainable from most Cambrian 

 shales in the Cordilleran region. Internally, however, these nodules 

 are peculiar as they consist of an odd mixture of brown and blue lime- 

 stone. Both sorts occur as distinct masses, sometimes sharply angular, 

 but more frequently irregular in shape, and the change from one to 

 the other is abrupt. The brown limestone, which looks like a fine 

 sandstone, is evidently rather pure calcium carbonate, judging from 

 its rapid effervescence, and, since this portion of the rock does not 

 scratch steel, it is assumed to be free from silica or sand. On the 

 other hand, the blue limestone masses contain sand grains or silica, 

 even though they also effervesce freely. Fossils are absent from the 

 brown masses but are very abundant in the blue portions. Strangely, 

 there are but two species of trilobites in these nodules, the abundant 

 Vanuxemella idahoensis and rare examples of Albertella sampsoni. 

 Neither species has been recognized in the larger shale fauna, although 

 elsewhere these trilobite forms are found together. 



