434 C. K. LEITH 



beds of the Flathead (Cambrian) sandstone come in contact with the 

 Belt rocks the dip and strike of the two are usually conformable, so far 

 as can be determined by measurement. This holds good all around 

 the great Belt Mountain uplift. It is only when contacts are examined 

 in detail, as near Helena, that the minor unconformities are discovered, 

 and only when comparisons are made between sections at some dis- 

 tance from one another that the extent of the unconformity becomes 

 apparent. In general from 3000 to 4000 feet of the upper strata of 

 the Belt terrane were removed by erosion in late Algonkian time before 

 the Middle Cambrian was deposited. It is believed that an uncon- 

 formity of this extent is sufficient to explain the absence of Lower 

 Cambrian rocks and fossils and to warrant the placing of the Belt ter- 

 rane in the Algonkian system. 



The fossils thus far found in the Belt terrane occur in the Greyson 

 shales at a horizon approximately 7700 feet beneath the summit of the 

 Belt terrane at its maximum development. The fauna includes four 

 species of annelid trails and a variety that appears to have been 

 made by a minute mollusk or crustacean. There also occur in the' 

 same shales thousands of fragments of one or more genera of crus- 

 taceans. 



Grand Canyon series. — The fossils of the Grand Canyon Upper 

 Cambrian series consist of specimens of a small discinoid shell found 

 in the upper division of the Chuar terrane, and a Stromatopora-like 

 form from the upper portion of the lower division and the central por- 

 tion of the upper division of the Chuar. Other obscure forms appear 

 whose identification is doubtful. 



In New Brunswick certain rocks below the middle Cambrian, 

 according to Matthew, contain fossils which may be pre-Cambrian. 

 (See summary of Matthew's articles, following, p. 435, and of later arti- 

 cle by Walcott, p. 436.) 



The Llano series of Texas is a series of alternating sandy shales, 

 sandstone, and limestone, very similar to those of the Grand Canyon 

 pre-Cambrian series and overlain by a middle Cambrian sandstone 

 similar to the Tonto sandstone of the Grand Canyon district. No fos- 

 sils have been found in these rocks, although no systematic search has 

 been made. 



The Avalon series of Newfoundland includes all the pre-Cambrian 

 sedimentary rocks of that area. Overlying them are Cambrian strata 

 carrying olenellus fauna. 



