164 



tion, transformed into silicates, such as serpentine, talc, chlorite and 

 pyroxene, by the subsequent intervention of heated solutions of alka- 

 line carbonates. 



It appears to me that we may explain the origin of these magne- 

 sian deposits, by the spontaneous evaporation of magnesian waters. 

 If the waters of Carlsbad were to become stagnant above their de- 

 posited travertine, they would yield by evaporation beds of ferru- 

 ginous dolomite, and waters like those of Caxton, Plantagenet, arid 

 Sainte-Genevieve would furnish carbonate of magnesia nearly free 

 from lime. Nothing forbids us to suppose the existence of waters 

 more highly charged than these with magnesian carbonate, formed 

 perhaps by the action of carbonate of soda upon lagoons of sea- 

 water, whose lime may be removed as carbonate, or by previous eva- 

 poration as sulphate. The lagoons in Bessarabia, supplied with the 

 waters of the Black Sea, deposit annually large beds of rock-salt ; 

 and it would require only the intervention of waters like those of the 

 natron lakes of Hungary and Egypt, to produce deposits of magne- 

 sian carbonate. 



The conditions of these deposits at Pointe-Levis and elsewhere in 

 the Quebec group, seem to point to the existence of basins along an 

 ancient sea-shore, which probably marked the first upheaval of the 

 older Silurian strata. Beds of travertine were there formed, and 

 then the sea flowed over these deposits and gave rise to the fossili- 

 ferous limestones ; but there were intervals of disturbance, indicated 

 by the conglomerates, and these movements, or the deposition of bars 

 along the shore, gave rise to lagoons or basins cut off from the sea, 

 where, by evaporation under the conditions which we have supposed, 

 magnesian precipitates would be deposited. The absence of fossils 

 in these beds is probably connected with the peculiar composition of 

 the waters. 



