MINERALS AND GEOLOGY OF CANADA. 216 
originally recognised: hence the separation of the so-called Middle 
Silurian series—these Anticosti beds being taken as the type of the 
latter subdivision. The expediency of the separation, however, is 
somewhat questionable. 
Finally, with regard to the Niagara Formation, it may be observed 
that limestone strata of apparently the same age, but resting on 
Huronian rocks, have been discovered at Lake Temiscamang, north of 
the great Laurentian water-shed which separates the northern geologi- 
cal area of Canada from the western and eastern areas of the south. 
See the general sketch of the distribution of our rock formations, a 
few pages further on. 
The Guelph Formation :—The rocks of this formation, unlike the 
Niagara and other Canadian strata, have not been traced beyond the 
limits of the Province. The ‘ Leclaire limestone”? of Iowa, which 
at one time was thought to belong to the same geological horizon, is 
now referred by Professor Hall to the Niagara subdivision. The 
Guelph Formation, as known in Canada, follows the more western 
limit of the Niagara area, and occurs especially in the vicinities of 
Galt and Guelph. According to Sir William Logan, it appears to 
form a lenticular-shaped mass, gradually thinning out both westward 
in Lake Huron, and in the neighbourhood of Ancaster, in the east. 
Its greatest thickness is estimated at about 160 feet. Its strata con- 
sist essentially of white or light-coloured dolomites mostly of a 
peculiar semi-crystalline or granular texture. These yield excellent 
building materials. 
Many of the enclosed fossils are identical with those of the Nia- 
gara beds, as Favosites Gothlandica (fig. 215), Halysites catenulatus 
(fig. 216), &c., but others appear to be confined to this formation. 
Fig. 225,—Casts of Megalomus Canadensis (Hall). 
