No. 1.] DAWSON — POST-PLIOCENE. 35 



and texture, it closely resembles the blue mud now in process of 

 deposition in the deeper parts of the Gulf of St. Lawrence. 



The lamination of the Leda clay and its included sand layers, 

 show that it was deposited at intervals, between which intervened 

 spaces when currents carried small quantities of sand over the 

 surface. In these intervals shells as well as sand were washed 

 over the bottom, while ordinarily Leda, Nucula and Astarte bur- 

 rowed in the clay itself. The layers and patches of stones I 

 attribute to deposit from floating ice, and to the same cause must 

 be attributed the large Laurentian boulders, occasionally though 

 rarely seen imbedded in the clay. 



The material of the Leda clay has been derived mainly from 

 the waste of the lower Silurian shales of the Quebec and Utica 

 groups, which occupy a great space in the basin of the Gulf and 

 River St. Lawrence. The driftage of this material has been to 

 the South-west, and in that direction it becomes thinner and finer 

 in texture. The supply of this mud, under the action of the 

 waves, of streams, of the arctic currents and tidal currents, and 

 floating ice, must have been constant, as it now is in the Gulf and 

 River St. Lawrence. It would be increased bv the melting of 

 the snows in spring and by any oscillations of level, and it is 

 probably in these ways that we should account for the alterna- 

 tions of layers in the deposit. 



The modern deposit in the Gulf of St. Lawrence, the chemical 

 characters and coloration of which I explained 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 mo- 

 dern 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 imperme- 

 ability, 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 of from twenty to one hundred fathoms in depth, corres- 



Journal of Geological Society of London, vol. v. pp. 25 to 30, 



