26o SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 64 



Algonkian and Cambrian sections at Helena and vicinity — Continued. page 



Cambrian basal unconformity _ 288 



Paleontological evidence 291 



Determination of genera and species by Rothpletz 293 



Comment on paleontological evidence 295 



Conclusions : 



Rothpletz 296, 297 



Walcott 296-298 



Some general conclusions 298 



ILLUSTRATIONS 



FIGURES 



10. Big Belt Mountains — Helena section, indicating unconformity between 



the Cambrian and pre-Cambrian 263 



11. Unconformity between Cambrian and pre-Cambrian in Grand Canyon, 



Arizona 283 



12. Unconformity between Helena limestone and Cambrian Flathead sand- 



stone • 286 



13. Unconformity between Marsh shale and Cambrian Flathead sandstone 286 



PLATES FACING PAGE 



39. Geologic map of area about Helena, Montana 262 



40 to 43. Unconformity between the Cambrian and pre-Cambrian in the 



Grand Canyon 282-288 



44. Brachiopoda from Park shale of Middle Cambrian 300 



INTRODUCTION 



In a recent memoir ^ on the fauna of the Belt formation near 

 Helena, Montana, Dr. August Rothpletz states that he found Lower 

 Cambrian fossils in a shale above a limestone which he identified as 

 the typical Helena limestone of the Belt series,^ and beneath another 

 limestone regarded by him as also a part of the Helena limestone. 

 He also states that in his four days' work about Helena he failed to 

 find any evidence of an unconformity at the base of the Cambrian 

 and therefore concludes that the Helena limestone and associated 

 formations of the Belt series are of Cambrian age and not pre- 

 Cambrian. 



Since my own extended examinations as well as those of other 

 geologists and paleontologists as to the position of the Helena lime- 

 stone and the age of the fossil fauna of the region lead to conclusions 

 very different from those of Doctor Rothpletz, it has seemed to me 



' Die Fauna der Beltformation bei Helena in Montana. ^Munich, 1915, pp. 

 1-46, with 3 plates. 



^ Bull. Geol. Soc. America, Vol. 10, 1899, Pre-Cambrian Fossiliferous Forma- 

 tions, by Charles D. Walcott, pp. 210-213. 



