SLAVT: river C0>X'RETI02fS 61* 



\ 

 its liver a notable quantity of mineral matter wouldj if buried in sedi- 

 ments, leave no visible trace, but its included mineral matter might 

 furnish a small invisible nucleus for the accumulation of other mineral 

 matter and thus become the nucleus of a concretion. It seems probable 

 that concretions in the Labrador clays and other clays discussed in this 

 paper without any evident nucleus may have started from an inorganic 

 nucleus accumulated in the tissues of a sheU-less mollusk which left no 

 recognizable trace in the sediments. This hypothesis must for the present 

 rest on the fact that a great many concretions have formed about definite 

 visible nuclei, which suggests that those without visible nuclei have not 

 becmn their crrowth without some initial determinant. This could have 

 been supplied by the mineral matter left in the sediments by the decay 

 of soft-bodied mollusks. 



Slave Eivee Coxceetioxs 



Concretions have been observed by the writer at only one point in the 

 Quaternary deposits of the ]S'orthwest Territories. 



Pleistocene clay overlaid by a deposit of sand about 100 feet thick out- 

 crops near Fort Smith, Just below the foot of the rapids. The clay at 

 this point contains numerous concretions which show some variety of 

 forms. The dominant form, however, is a compound discoid type com- 

 prising a series of regularly graded disks, which give the sides of the 

 concretion a terraced or stair-step aspect. The concretions are not 

 sharply truncated above or below, but have the top and basal sides round- 

 ing off gently into dome-shaped surfaces, ^o organic remains have been 

 found in the clay which furnishes the Slave Eiver concretions, and the 

 concretions are, so far as known, without any visible nuclei. This ter- 

 raced concretion represents a tjipe of form which has not been observed 

 among the Pleistocene concretions from eastern Canada, The clay out- 

 crop from which they are derived is not sufficiently well exposed to show 

 the laminae which are assumed to be present and to account for the 

 terrace-like divisions seen on the concretions. 



The clays furnishing these concretions have been observed only at the 

 Fort Smith locality. The sands which follow above them are apparently 

 without the concretions which characterize similar sands in the Ottawa 

 Valley. 



British Columbia Coxcretioxs 



The compoimd discoid type of concretions is represented by remark- 

 ably perfect and beautiful specimens from Quaternary clays at Endako. 



