STRATIGRAPHY 



317 



Algonkiau. 



- Altyn limestone. 



f Limestone, of which two members are 

 distinguished ; an upper member of 

 argillaceous, ferruginous limestone, 

 yellow, terracotta, brown, and garnet 

 red,very thin-bedded ; thickness, about 

 600 feet; well exposed in summit of 

 Chief mountain (figure 1, plate 52) ; 

 and a lower member of massive lime- 

 stone, grayish blue, heavy-bedded, 

 somewhat silicious, with many flat- 

 tened concretions, rarely but definitely 

 fossiliferous ; thickness, about 800 feet ; 

 type locality, basal cliffs of Appekunny 

 mountains, north of Altyn, Swift Cur- 



[ rent valley (figure 2, plate 47). 



ALGONKIAN 



Correlation. — The oldest rocks found in this district are those which 

 constitute the Lewis and Livingston ranges. The oldest formation of 

 the series, the Altyn limestone, is assigned to the Algonkian period on 

 the basis of fossils discovered by Weller in its characteristic occurrence 

 at the foot of Appekunny mountain near Altyn, Montana. These fos- 

 sils are fragments of very thin shells of crustaceans. They have been 

 examined by Walcott, who states : 



"The fragments of crustaceans collected by Professor Stuart Weller, in Montana, 

 may be referred provisionally to Beltina danai, as described in volume X, page 

 338, of the Bulletin of the Geological Society of America. 



"The mode of occurrence of the material is similar to that found in the Grayson 

 shales of the Algonkian in the Belt mountains, Montana. Hundreds of broken 

 fragments of the carapace of the crustaceans are distributed unevenly through the 

 rock. Occasionally a segment or fragment of what appears to be one of the ap- 

 pendages is sufficiently well preserved to identify it." 



The fossiliferous strata of the Belt formation in the Belt range are 

 separated from the Cambrian by 7,700 feet of sediments and an extensive 

 unconformity. In the Front range of the Rockies 10,700 feet of appar- 

 ently conformable strata overlie the fossiliferous bed, and it is possible 

 that the plane of division between Algonkian and Cambrian as deter, 

 mined by paleontologic evidence will be found in this great series. In 

 the upper part of the Siyeh limestone near the head of Mineral creek 

 Weller found some indistinct forms which he considers possibly to be 

 parts of crustaceans. Walcott expresses a similar view, saying : 



"Mr Weller's suggestion that the fragments possibly represent crustacean re- 

 mains appears to be the most plausible. If from a Devonian horizon they would 

 suggest the genus Licas, or some of its subgenera. It is a case where more mate- 

 rial is needed in order to arrive at any definite conclusion." 



