CAMBRIAN GEOLOGY AND PALEONTOLOGY 



III 



No. 4.— RELATIONS BETWEEN THE CAMBRIAN AND 



PRE-CAAIBRIAN FORMATIONS IN THE ATCINITY 



OF HELENA, MONTANA 



By CHARLES D. WALCOTT 

 (With Plates 39 to 44) 



CONTENTS PAGE 



Introduction 260 



Walcott's diagrammatic section of 1809 262 



Algonkian and Cambrian sections at Helena and vicinity 265 



Pre-Cambrian Algonkian formations 265 



Spokane shale 265 



Empire shale 265 



Helena limestone 266 



Marsh shale 267 



Cambrian formations 267 



Flathead quartzite 267 



Wolsey shale 268 



Meagher limestone 268 



Park shale 268 



Pilgrim limestone 269 



Dry Creek shale 269 



Yogo limestone 269 



Summit of the Cambrian 269 



Cambrian section to the northeast of Helena on Beaver Creek 270 



Cambrian section of the Little Belt Mountains 273 



Stratigraphic section of the Big Snowy Mountains 273 



Rothpletz section of Mount Helena 2T] 



Confusion in identification of the pre-Cambrian Helena limestone. 279 

 Rothpletz interpretation of Weed's Cambrian as given in Butte 



report 281 



Comparison of Rothpletz and Weed Cambrian sections in the 



Helena District 281 



Unconformity between the Cambrian and pre-Cambrian 282 



Grand Canyon section 282 



Bow River Valley section 282 



Belt Mountains — Helena section 285 



Extent and character of recognized contacts 285 



Explanation of apparent conformity 287 



Extent of unconformity 288 



Epeirogenetic unconformities 288 



Smithsonian Miscellaneous Collections, Vol. 64 No. 4 



259 



