246 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



are rare, although much of the rock is made up of comminuted fragments of organic remains. 

 The thicker beds are rendered somewhat porous by the numerous small cavities left between 

 these fragments. Most of the beds show a crystalline character on fresh fracture. 



On the south end of FitzwiUiam Island and on the eastern part of the south shore of Mani- 

 toulia Island there is a thickness of about 100 feet of heavy-bedded, coarsely spongy gray and 

 buff dolomite, which contains a few fossils like those of the Guelph formation. 



Monroe is a term formerly applied by Lane to all the Silurian above the " Niag- 

 ara" (that is, between the "Niagara" and the Dundee limestone), as he found it 

 impracticable to separate the strata consistently. Although a division can not be 

 made distinctly in the field, Lane ^" now recognizes the following subdivisions, in 

 ascending order: Salina, or Lower Monroe; Bass Islands series, or Lower Monroe [in 

 the restricted sense]; Sylvania sandstone, or Middle Monroe; and Detroit River series, 

 or Upper Monroe. He discusses their thickness and character at length. 



The "Salina" he defines as extending from the top of the "Niagara" to the 

 uppermost salt bed. The base of the "Salina" should be marked, according to 

 Grabau, as quoted by Lane, by a hiatus and " disconformity. " According to Lane 

 the strata thicken greatly northward from Monroe County, presumably by the addi- 

 tion of beds at the bottom in the deeper part of the basin. At Milan rock salt seems to 

 occur almost directly above the "Niagara," whereas at Wyandotte there is 275 feet 

 of dolomite below the rock salt and above the white dolomite. 



The top of the " Salina" Lane would determine by measuring from the Sylvania 

 sandstone down to the top of the first salt bed. He would then class aU the beds 

 between that stratum and the "Niagara" below as "Salina." He thus obtains 

 thicknesses for the " Salina" which range from 370 feet (at Britton) to 959+ feet (at 

 Royal Oak). Inasmuch, however, as the salt bed thus determined is by no means 

 certainly at one horizon, and gypsum if not salt occurs above the Sylvania sandstone, 

 the division is very indefinite and of little stratigraphic significance. 



Although the top of the "Salina" is thus difficult to determine, it is made the 

 base of a division which comprises all the Silurian above the " Salina" and which in 

 distinction to that formation is strictly marine. Unfortunately the name " Monroe" 

 has been given to this division in a limited sense excluding the " Salina, " in contrast 

 to the broader definition including "Salina," which has become established. This 

 limited "Monroe" has been subdivided into three parts and the upper and lower of 

 these each again into four members. Lane, Prosser, Sherzer, and Grabau*^* have 

 published the following : 



MONROE FOEMATION. 



The highest SUuric strata of America are represented by the Monroe formation of Michigan, 

 using this term in its limited sense for the post-Salina SUuric. The Monroe formation is a 

 strictly marine series succeeding the abnormal (nonmariae [Grabau]) Salina. It is traceable 

 throughout lower Michigan westward into Wisconsia and eastward through Ohio, Ontario, and 

 into western New York. Where recognizable a strong dividing line separates the lower from the 

 upper part, and in general this division is emphasized by the occurrence of the Sylvania sand- 

 stone, a rock of pure quartz grains, and representing, according to Grabau and Sherzer, an 

 eolian deposit formed under conditions similar to those now found in the Libyan Desert. Accord- 

 ing to this interpretation, the Sylvania sandstone represents a long time interval the depositional 

 marine equivalent of which is stiU unknown. This long interval is further emphasized by the 

 entire distinctness of the faunse of the lower and upper divisions of the Monroe. 



