>58 



Transactions of the Canadian Institute. 



[Vol, VII. 



there is a merging of conditions there will be a merging of forms. 

 Accidents may happen by which the normal distribution is slightly 

 disturbed ; and some few forms may exist in all three zones. 



Since the production of the deposits, their thickness, and their other 

 relations depend upon the two factors, rate of depression and rate of 

 supply of detritus, with varying conditions as to depth of water, there 

 are many possible variations from the normal arrangement. The result 

 of one such variation is represented diagramitically in figure lo, where 

 the rate of supply of detritus has been sufficiently in excess of the rate 



Sea Level 



Figure io. — Diagram to repiesent a special variation in the distribution of sediments. A, oldland ; 

 B, sandstone and conglomerate zone ; C, shale zone ; D, limestone zone. Transition zones are indicated by 

 lines, he, sub-aerial deposits. 



of depression of the land, to permit of the transgression of the sands 

 over the seaward zones. If a rapid variation had taken place in the 

 opposite direction, the muds and sands might become mingled along the 

 shore, and eventually the limestones might rest directly upon the 

 oldland surface. Irregularities in the variations of each factor will lead 

 to many irregularities and overlaps along the zones of transition. 



By subsequent processes, after a greater or less interval of time, 

 these deposits will become indurated and form sandstones and conglom- 

 erates, shales, and limestones. If, after uplift, the greater part of the 

 sediments are eroded away, and small remnants, perhaps as outliers, 

 remain in protected areas, we may find limestone in one place which is 

 contemporaneous with sandstone in another, though the fossils in each 

 are wholly unlike. Moreover, deposits, of entirely different epochs, may 

 be almost identical because derived from the same source. 



Sandstones. — In Central Ontario, particularly towards the eastern 

 end, where the sediments occur in ellipsoidal basins, and with a very 

 slight and irregular dip, the sandstones and some of the other beds 

 entirely without fossils, there is a strong a priori argument for consider- 

 ing that the sandstones and some of the limestones are contemporaneous. 

 The oldest sediments, within the area, which can be identified by fossils, 

 are the Black River limestones. Conformably below these are beds, 



