54 THE ICE AGE IN CANADA. 



many years ago,* shows us that the Leda clay, when in 

 suspension, was probably reddish or brown mud tinted 

 with peroxide of iron, like that which we now see in the 

 lower St. Lawrence ; but like the modern mud, so soon as 

 deposited in the bottom, the ferruginous colouring matter 

 would, in ordinary circumstances, be deoxidised by organic 

 substances, and reduced to the condition of sulphide or 

 carbonate of the protoxide. This colour, owing to its 

 impermeability, it still retains when elevated out of the 

 sea ; but when heated in presence of air, or exposed for 

 some time at the surface, it becomes red or brown. The 

 occasional layers of reddish Leda clay indicate places or 

 times when the supply of organic matter was insufficient 

 to deoxidise the iron present in the mass. 



The greater part of the Leda clay was probably 

 deposited in water from twenty to one hundred fathoms 

 in depth, corresponding to the ordinary depths of the 

 present gulf of St. Lawrence ; and as we shall find, this 

 view is confirmed by the prevalent fossils contained in it, 

 more especially the Foraminifera. The most abundant 

 of these in the Leda clay is Polystomella striatopunctata 

 var. arctica, which is now most abundant at about twenty- 

 five or thirty fathoms. Since, however, the shallow-water 

 marine Post-pliocene beds extend upwards in some places 

 to a height of six hundred feet on the hills on the north 

 side of the St. Lawrence, it is probable that deposits of 

 Leda clay contemporaneous with these high-level marine 

 beds were formed in the lower parts of the plain at depths 

 exceeding one hundred fathoms. 



The western limits of the Leda clay appear to occur 

 where the Laurentian ridge of the Thousand Islands 



* Journal of Geological Society of London, Vol. V. , pp. 25 to 30. 



