634 E. M. IvIXDLE CANADIAN PLEISTOCENE CONCRETIONS 



development and (2) by the occnrrence of certain local conditions essen- 

 tial to tlieir formation. 



These types include the compound discoid forms found in British 

 Columbia and on the Slave Eiver in the Northwest Territories, which 

 are unknown in eastern Canada. The claystones of the lower clays in the 

 Ottawa Valley and eastern Labrador belong to another type characterized 

 by a great variety of form, but showing a generic resemblance in the 

 smooth surface which is always present and the approach to symmetry 

 which these concretions exhibit. A third type, which has been desig- 

 nated as marlehor, contrasts sharply with the approximately symmetrical 

 smooth-surfaced claystones. The ma^'lekor are distinctly unsymmetrical 

 and possess a highly irregular surface and internal features never found 

 in the claystones. When found in the same district, the marlehor occur 

 above the claystones in the area studied. 



Another group of concretions, with only a slight resemblance to the 

 preceding, are grouped under the class name of rhizoconcretions. These 

 comprise three distinct varieties of concretions, each of which has a root 

 for a nucleus. One of these is confined to the sands which lie at the top 

 of the Ottawa Valley Pleistocene section. Another variety is confined to 

 the upper clay beds of the same section. The third is found in the upper 

 part of the beds in which the marlekor occur. None of these rhizocon- 

 cretions is known in the lower clays of the section in which the clay- 

 stones are so abundant. 



Certain irregular-shaped, slender, vertical structures of soft clay pene- 

 trating a sand formation are described as pseudoconcretions. These 

 occur in the Ottawa section and are considered to be clay-filled burrows 

 of marine animals. 



Some of the concretions discussed in this paper are known to be con- 

 fined to the superficial portions of the beds in which they occur. All of 

 them are probably limited to those parts of the formation containing 

 them which lie relatively near the surface. 



Calcium carbonate or other mineral matter separated from the sea- 

 water and segregated in the sediments by the decay of soft-bodied marine 

 invertebrates may have supphed nuclei in many cases for concretions 

 without visible nuclei. 



Discussion 



Prof. T. T. QuiRKE : Certain surfigial features on marlekor look like 

 welts or small, raised ridges. They may have been caused by the filHng 

 of shrinkage cracks by hardened material, followed by continued shrink- 

 age of the main mass. Other concretion-like masses appear to have been 



