130 INDEX TO THE STEATIGEAPHY OF NOETH AMEEICA. 



the identity of the Flathead is clear, knd the limestone of the Eed Lion is probably equivalent 

 to the Yogo, the highest Cambrian formation of the central Montana sections. 



The age of the intermediate members of the series is more obscure, but tentative correla- 

 tion may be based on the following considerations : There is in the Philipsburg Cambrian and 

 in that of the Little Belt Mountains and the Livingston region a basal sandstone or quartzite, 

 overlain by alternating shales and limestones that constitute more or less definite stratigraphic 

 units. It is a reasonable supposition that considering only relatively thick and homogeneous 

 strata, the lowest shale in the Philipsburg section corresponds to the lowest shale in the Liv- 

 ingston section, the lowest limestone in the one to the lowest limestone in the other, and so on. 

 On this assumption, the most probable correlation seems to be the following: 



Tentative correlation of Cambrian formations. 



Phlllpsbuig quadrangle. Little Belt Mountains. 



T, jT- c .■ f Upper member Yogo limestone. 



Keel liion formation w , -n. /-. i i, i 



1 Lower member Dry Creek shale. 



Hasmark formation Pilgrim limestone. 



{Shale and laminated limestone Park shale. 

 Laminated limestone Meagher limestone. 

 Green shale Wolsey shale. 



Flathead quartzite Flathead quartzite. 



There is not, however, a sufficiently detailed resemblance between the Philipsburg section 

 and those farther east to make a correlation so largely lithologic whoUy safe. The Pilgrim 

 limestone, for example, has not been described as magnesian and had it been as obviously so 

 as the Hasmark, the fact would hardly have escaped notice ; the few specimens in the National 

 Museum do not appear magnesian. The pecuhar laminated limestone of the Eed Lion forma^ 

 tion, again, does not seem to resemble the Yogo. A real correlation between the Philipsburg 

 and other sections is hardly possible before more Cambrian fossils have been found about 

 Philipsburg, and strata actually traced from one region to another. There is reason to believe 

 that when both these things have been accomplished, formations equivalent in the usual senses 

 may prove to be not quite contemporaneous. The highest hmestone in the Philipsburg quad- 

 rangle has a fauna of younger facies than that in the Little Belt Mountains, and the Flathead 

 quartzite contains older fossils in the more easterly parts of Montana than in the more westerly 

 parts. It may be that for each or for several of the epochs, of the Cambrian period the con- 

 trolling conditions of deposition were initiated at the east and gradually invaded more and 

 more westerly areas — that there were, in fact, successive westward transgressions. 



li 18-19. ST. LAWRENCE VALLEY. 



In the valley of the St. Lawrence it is necessary to distinguish the forma- 

 tions of the Ottawa Basin and its northeastern extension to Quebec from those of 

 the region lying farther southeast. The two are separated by the St. Lawrence- 

 Champlain thrust and the stratigraphic sequences are unlike, as are the facies of the 

 formations and faunas. (See Chapter IV, pp. 196-206.) 



In the Ottawa Basin and thence past Montreal the Cambrian is represented only 

 by the overlapping littoral deposit of the Potsdam (Upper Cambrian) sandstone. 

 The Lower Ordovician Beekmantown or " Calciferous " succeeds. The characteristics 

 of these formations and the enumeration of their fossils are given according to Ami 

 in the tables quoted in Chapter IV (pp. 200 and 201). 



Logan's original descriptions^** remain the most satisfactory concerning the 

 details of character and occurrence in Canada. In so far as they relate to the 

 stratigraphy of the formations in New York, they are replaced by Cushing's recent 

 work. (See pp. 120-123.) 



