238 On theComposition and Metamorphoses of Sedimentary Rocks. 



The post-pliocene clays of the St. Lawrence valley also eon- 

 tain a large proportion of alkalies ; they are sometimes calcareous, 

 and afford, besides carbonates of these bases, a considerable 

 amount of lime and magnesia as silicates. The following results 

 were obtained with a reddish-fawn coloured clay from the Riviere 

 k la Graisse, Rigaud ; it is impalpable, and remains a long time 

 suspended in water. It yields to heated hydrochloric acid 12'95 

 per cent, of alumina and peroxide of iron, 3"97 of lime and 1'92 

 of magnesia ; the amount of carbonic acid evolved is a little more 

 than is required to form a carbonate with the lime thus obtained. 

 Its complete analysis gave — 



Silica (by difference) . . . 50"81 



Alumina 21-70 



Peroxide of iron 5*60 



Lime 5*32 



Magnesia 2-62 



Potash 2-85 



Soda 2-61 



Phosphoric acid "74 



Carbonic acid 3*25 



Water 4-50 



100-00 



Another clay from the vicinity of Montreal, which was mixed 

 vidth 13-5 per cent, of siliceous sand and a little magnetic iron, 

 and was free from earthy carbonates, afforded (the sand included) 

 the following results : — 



Silica (by difference) 65-53 



Alumina 13-15 



Peroxide of iron 8-50 



Lime 1-73 



Magnesia 1-14 



Potash 1-76 



Soda 2-35 



Phosphoric acid -54 



Water and some organic matter . 5-30 



10000 



The first of these clays, in the proportions of silica, alumina, 

 lime and alkalies, approaches some of the lime felspars, and its 

 metamorphosis might yield a mixtui'e of andesite or labradorite, 

 with hy]:)ersthcne, constituting a rock similar to one which is 

 very abundant in our older ante- Silurian strata, and may very 

 well have been the source of this clay. 



Montreal, Canada, Feb. 20, 1854. 



