REPORT OF TEE CEIEF ASTRONOMER 181 



SESSIONAL PAPER No. 25a 



formation in the Belt range are separated from the Cambrian by 7,Y00 

 feet of sediments and an extensive unconformity. In the Front range of 

 the Rockies 10,700 feet of apparently conformable strata overlie the fossili- 

 ferous bed, and it is possible that the plane of division between the Algon- 

 kian and Cambrian as determined 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 con- 

 siders as possibly to be parts of crustaceans. Walcott expresses a similar 

 view, saying: 



' " Mr. Weller's suggestion that the fragments possibly represent crusta- 

 cean remains 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 material is needed in order to arrive at any definite conclu- 

 sion." ' 



In his paper on •' Algonkian Formations of Northwestern Montana' Wal- 

 cott refers the entire Lewis series and its equivalents to the pre-Cambrian 

 (Algonkian) system.f In this conclusion he has been followed by Calkins, 

 Eansome, MacDonald, and Lindgren, all working on the western phase of the 

 Belt terrane in Idaho. § The same view has governed the compiling of the 

 geological map of North America which was prepared for the session of the 

 International Geological Congress held at Mexico City in 1906; the large area 

 of ' Neo-Algonkian ' shown in the States of Montana and Idaho represents the 

 Belt terrane. 



As one of Walcott's last (1906) papers on this subject shows the trend of 

 opinion among the United States geologists, the more important parts of his 

 table of equivalents has been reproduced in Table IX. of the present report. 



* B. Willis, Bull. Geol. Soc. America, Vol. 13, 1902, p. 317. 



t C. D. Walcott, Bull. Geol. Soc. America, Vol. 17. 1906, p. 17. 



§ W. Lindgren, U.S. Geol. Surv., Prof. Paper, No. 27, 1904, p. 16. 



F. L. Ransome, U.S. Geol. Surv., Bull. No. 260, 1904, p. 277. 



D. F. MacDonald, U.S. Geol. Surv., Bull. 285, 1906, p. 43. 



F. C. Calkins, U.S. Geol. Surv. Bull. 384, 1909, p. 27 



