SILURIAN. 245' 



latter are not uniformly extensive. In Canada they include the "Onondaga Salt 

 group" and the "Water lime" (Bertie or Cayuga dolomite of Chapman ^^°), which 

 was referred by Logan *''"' '**"' ^^^ to the "Lower Helderberg" but is now regarded 

 by Clarke as a member of the Salina formation. 



In New York the Guelph dolomite, the uppermost formation of the Niagara in 

 Canada, has a restricted extent only.^^* The "Onondaga Salt group" (called 

 Salina by Dana *^^) and the "Water lime" are now included in the Cayugan, the 

 recognized subdivisions ^^ of which are known as Salina formation, Cobleskill lime- 

 stone, Rondout water lime, and Manlius limestone. Their relations and the western 

 representatives of the formations of New York are discussed by Grabau.^^ In this 

 part of Canada the "Lower Helderberg" of Logan and Chapman comprises only a 

 part of the " Water lime" which in central New York underlies the strata which were 

 formerly known as "Lower Helderberg" but which are now called Helderberg or 

 Helderbergian and assigned to the Devonian. ^^^* Merrill, whose geologic map of the 

 State *^^ has (generally) been followed in the map of North America, placed the line 

 between Silurian and Devonian at the top of the "Lower Helderberg." The North 

 America map of 1906 followed this delineation. The present map does not depart 

 therefrom, for the scale is too small to show the difference, but the Helderberg 

 ("Lower Helderberg") is included in the Devonian in the citations contained in this 

 volume.^*' **^' ^"^' "^^ 



L 16-17. NORTHERN MICHIGAN AND ONTARIO. 



There seems to be some question as to the presence of the Clinton in Michigan. 

 Certain beds which have been referred to by several authors as Clinton are, in the 

 opinion of ULrich, more likely to prove of Richmond age. Aside from these doubtful 

 beds the Silurian consists of the dolomitic limestones commonly referred to the 

 Niagara and the divisions which have been grouped by Lane and others under 

 the term Monroe. Of the Niagara as mapped by the Michigan State Survey and 

 others Lane ^" recognizes the following subdivisions: Clinton, to 130 feet; 

 Rochester shale, doubtfully identified, 20 to 30 feet; Lockport and Guelph dolo- 

 mites, 270 to 600 feet, the greater thickness being in the northern part of the State. 

 The two latter Lane combines under the name Manitoulin. 



Rominger "^ describes the "Niagara" of the Upper Peninsula of Michigan in 

 detail for different localities and sums up the section as follows : 



The rock beds of the Niagara group allow a subdivision in three well-marked sections. It is 

 exclusively a limestone formation. The lower section is always very regular and even-bedded, 

 composed of comparatively thin layers of a fine crystalliae graia, or with a duU, more earthy 

 fracture. In composition most of the strata are dolomites ; only a few layers are found to be a 

 pure limestone. Fossils are rare in it. The middle division is made up by more massive, highly 

 crystalline dolomite ledges, which usually contain a large number of the casts of the Pentamerus 

 ohlongus and some ill-preserved corals. The upper division is a series of thin, uneven layers, with 

 iatermixture of much siliceous matter with the dolomite mass, and of seams and nodular concre- 

 tions of hornstone. In this upper division, also, the greatest abundance of fossils is found. 



With reference to the " Niagara" of Manitoulin Island, Bell "^ states: 



, The southern portion of the peninsula between South Bay and the eastern side of Manitoulia 

 Island belongs to the Niagara formation, which has here a thickness of about 405 feet. It con- 

 sists principally of heavy-bedded light-gray, light bluish-gray, and buff dolomites. Good fossils 



