igoo-i.] Physical Geology of Central Ontario. i6i 



sion of the land. That it did not cease is shown by the thickness of the 

 deposits. 



Throughout the Lorraine epoch the area has been one of large 

 sandy mud flats, alternately bare, exposed to the sun and rains, and 

 submerged. The shallow sea appears to have endured for some time, 

 since these deposits gradually give place to the sandstones of the 

 Medina epoch. During the Medina there has been an alternation of 

 the depth of the ocean here, as evidenced by the mingled sandstones 

 and marls, the former with mud cracks, ripple and current marks. The 

 final stage of the Medina led to the accumulation of a broad thick band 

 of fine-grained siliceous sandstone, free from ferric oxide, in marked 

 contrast to the majority of the lower beds. 



The succeeding epoch must have begun with a relatively rapid 

 depression of the land, since the overlap of the Clinton dolomitic lime- 

 stone upon the upper sandstone of the Medina is abrupt. The depres- 

 sion seems to have continued rapidly enough to permit of the overlap 

 of the succeeding Niagara rocks upon the crystalline areas far to the 

 north. From the purity of the limestone, and from the types of organic 

 remains, and their abundance, it is inferred that the waters of this epoch 

 were clear and warm. The materials from which the limestone is made 

 were probably drawn from the sea water by the invertebrate animals in 

 the making of their hard parts. 



This second great limestone-making epoch was followed by a 

 gradual shallowing of the water, during which the Guelph dolomites 

 were formed. Eventually the water became very shallow ; enclosed 

 lagoons, occasionally flooded, were numerous ; in these lagoons the salt 

 and gypsum beds of the Onondaga were formed by the evaporation of 

 the water and the concentration and precipitation of the saline 

 compounds in solution. 



The sandstones of the succeeding Devonian period are now many 

 miles distant from the front of the Niagara cuesta. They may at one 

 time have reached out and overlapped it, but if so, what their north- 

 eastern extension may have been is unknown. During the period of 

 their formation the central portion of the Archean area may have been 

 above water, and the denudation which has subsequently removed all 

 the Niagara limestone, with a very few small protected areas excepted, 

 could then have already begun. It is interesting to note that the 

 peneplain represented by BC (figure i,page 144) may date its beginning 

 from this Devonian degradation. 



