144 THE ROYAL SOCIETY OF CANADA 



but have subsequently been removed by erosion. The marl is fresh- 

 looking and, where not rust stained from overlying deposits, it is 

 yellow-white to white. A block of the material placed in water dis- 

 integrates rapidly. It consists of a large proportion of freshwater 

 shells well preserved in a matrix of a fine powder of calcium carbonate. 

 Only a few shells are fragmentary; for the most part they are complete. 

 There have been two theories advanced with regard to marl 

 deposition. The older one assumes that the lime is precipitated 

 chemically owing to the removal of carbon dioxide from the water. 

 Davis, in two papers,^ has shown that certain algae, particularly 

 Chara and to a lesser degree Zonotrichia, play an important role in 

 the formation of marl, the latter forming incrustations above the 

 stems of the algae. Chara is present in the waters of McKay Lake 

 at the present time, where marl deposition is very small. No evi- 

 dence has been procured, however, to show that it was present in the 

 early stages of the lake at the time the marl was deposited. The fine 

 material shows no evidence of having formed an incrustation on plant 

 stems, no- tubes of calcium carbonate which surround the stems are to 

 be found and no oogonia or fruit bodies of the plant are present. It 

 must be remembered that this material was not subjected to any 

 wave action and hence it is unlikely that such material would be 

 completely destroyed if originally present. So it seems probable in 

 this particular instance the marl has been largely produced by dis- 

 integration of molluscan shells and by chemical precipitation from 

 solution. 



The most probable source of the marl in this deposit would be 

 the lime content of the marine clays which were leached out and 

 carried into the waters of the former McKay Lake when its size was 

 much greater than now. In some other parts of the Ottawa Valley 

 instead of forming marl beds the lime content of the marine clays has 

 leached down and cemented the underlying gravels. This is well 

 shown at Tenaga, Quebec. The calcium carbonate in the marl at 

 McKay Lake shows no tendency to work down and cement the under- 

 lying sands. 



These marls are probably contemporaneous with the beds in the 

 section on p. 150, immediately above the slaty gray clay. As in 

 present day deposition the marl was deposited around the shore while 

 sediments of a different nature were deposited in the deeper portions 

 of the lake. 



iDavis, C. A. A Contribution to the Natural History of Marl: Jour, of 

 Geology, Vol. VIII, 1900, pp. 485-497, and, A Second Contribution to the Natural 

 History of Marl: Jour, of Geology, Vol. IX, pp. 491-506. 



