OTTAWA VALLEY CONCRETIONS 623 



tish type of concretion's present many erratic shapes, some of which have 

 numerous sharp-pointed extensions. Forms with lateral extensions ter- 

 minating in sharp j^oints, the flattened doughnut type, and various other 

 irregular shapes generally sliow a certain amount of symmetry with ref- 

 erence to a plane midway between the upper and lower sides. The irreg- 

 ular shape of such concretions is probably due to slight different degrees 

 of resistance to their growth in horizontal directions, growth taking in 

 such cases always the line of least resistance. This resistance, if we may 

 judge from the symmetry of the concretions, has varied but little in the 

 \ertical direction, but greatly in the horizontal direction. Many concre- 

 tions have their symmetry interrupted through attachment to or partial 

 inclosure of a fossil shell or pebble. In such cases the attached shell or 

 pebble has probably served as the nucleus or starting point of the concre- 

 tion, but conditions of growth have been more favorable on one side of 

 tlie nucleus than on others, thus leaving the pebble or shell nucleus on the 

 periphery instead of at the center of the concretion, where it is often found 

 (plate 10, figure 7) . The rough surface shown by figures 6 and 8, plate 10, 

 is the result of the clay which adheres to the concretions when remoAcd 

 from the original bed. Weathering leaves such surfaces entirely smooth. 

 Specimens occur but rather rarely in which the concretionary growth 

 has centered about a very angular rock. A highly angular piece of lime- 

 stone serves as the nucleus of the concretionary growth shown in plate 9. 

 In other cases small shells are found on the periphery of greatly depressed 

 oblate spheroids, whose position suggests that their inclusion within the 

 limits of the concretion has been a purely accidental matter, resulting 

 from their having been within the limits of the increasing radius of the 

 growing concretion (plate 6, figure 3). In such cases the true nucleus 

 may be of the type suggested in discussing Labrador concretions. 



In spite of the great diversity in form shown by the concretions of this 

 class, which may be designated as claystones, they show, with very few 

 exceptions, some form of symmetry or an approach to symmetry, and 

 always the surface of weathered specimens exhibits a more or less regular 

 curvature. These two characteristics distinguish them quite clearly from 

 the concretions of the second group of sediments in the Pleistocene de- 

 posits of the Ottawa Valley. 



One of the most interesting and significant features connected with the 

 Pleistocene concretions is seen in their distribution. The distrilmtion of 

 the concretions of the lower clays and of the other divisions to he con- 

 sidered are all sharply localized. Concretions may be found s]:)aringlv or 

 rather rarely at most places along the rivers and creeks in tlie lower 

 marine clay, but they occur abundantly at only a few places. In sharp 



