54 'I'TIK IC'K A(iK L\ CANADA. 



iiiimy yoiirs a^o,* sIkiws us that I Ik; lA'da clay, wlicn in 

 Kn.s|K'iisi()ii, was jJidliaUly reddish or lirnwii mud tinted 

 with })en).\ide of ii'on, like, that whieh we now .see in the 

 lower St. Lawrence; but like the modern mud, so so(»n as 

 deposited in the l)f)ttom, the ferruginous colouring matter 

 would, in ordinary circumstances, be deoxidised by organic 

 substances, and reduced to the condition of sulphith^ or 

 carbonate of the protoxide. This colour, owing to its 

 inii)ermeubility, it still retains when ele\'ated out of the 

 sea; but wiien heated in presence of air, or expo.sed for 

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

 occasional layers of reddish I.eda, clay indicate places or 

 thues when the su})ply of organic matter was iusutlicient 

 to deoxidise the iron present in the mass. 



The greater part of the Leda clay was probal)ly 

 deposited in water from twenty to one hundred fathoms 

 in depth, corrcs])()nding to the ordinary de})ths of the 

 present gulf of St. Law]'ence ; and as we shall tiiul, this 

 view is conHrmed by the prevalent fossils contained in it, 

 more especially the Foraminifera. The most abundant 

 of these in the Leda clay is ]'(i/i/>if(iiiic//a slridtopunrtafa 

 \'A\\ur(fir(i,\\\\\c\\ is now most abundant at about twenty- 

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

 marine rost-])liocene beds extend upwards in some i)laces 

 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 [)arts of the plain at de])ths 

 exceeding one hundred fathoms. 



The western limits of the Leda clay appear to occur 

 wliere the Laurentian ridge of the Thousand Islands 



.Journal of (ieological Society of Loiuion, Vol. V., pp. 25 to W. 



