018 E. M. KIXDLE CAXADIAX PLEISTOCENE CONCRETIONS 



British Columbia, which were brought to the writer's notice by Mr. C. J. 

 Kettyle. Mr. Kettyle, who generously collected and presented a collection 

 to the Canadian Geological Survey, later obtained and sent the Survey, 

 at the writer's suggestion, samples of the inclosing marl-like clay^^ in 

 which these concretions formed. Four specimens of these concretions are 

 figured on plate T, tigures 3 to 6. Specimen of the inclosing clay sent 

 l)y ^Ir. Kettyle show fine lamination, and the superposed disks of which 

 tlie concretions are composed appear to correspond in thickness with the 

 lamina? of the beds in which they formed. 



The Slave Eiver concretions contrast with the Endako concretions in 

 the uniformly decreasing size of the disks of the former, each one being, 

 like the units in a pile of scale weights, smaller than the preceding. The 

 disks of the British Columbia concretions in many specimens fluctuate 

 al)ruptly in size, repeatedly increasing and then decreasing in diameter 

 in the same specimen (plate 7). 



In this collection the operation of the forces tending to produce radial 

 symmetry in concretion growth have reached a degree of perfection in 

 the resulting forms which is seldom equaled. The perfect circles repre- 

 sented by the disks of constantly changing size piled one upon another 

 to the number often of one or two dozen show the radial symmetry in a 

 most striking way. In the top-shaped form (plate T. figures 3-6), with 

 its seventeen disks of sharply contrasted size, the periphery of each is a 

 perfect circle. In other forms which exhibit a regularly changing diam- 

 eter tlie peripheries of the cross-sections represent perfect ellipses. In 

 certain other more complicated tigures the periphery of some of the sub- 

 (hscoid elements will take at the two ends of the major axis a "V'-shaped 

 dip. thus preserving perfect symmetry for the particular section involved. 

 In one of these ellii^soidal concretions two wart-like knobs are placed 

 precisely at the ends of the long axis of the elliptic zone to which they 

 belong. 



These beautifully symmetrical forms invite some consideration of tbe 

 ])r()bab]e reasons for their development. On theoretical grounds a com- 

 pletely homogeneous matrix, furnishing conditions in wliich the physical 

 resistance to concretionary growth is of uniform character and of mod- 

 erate or minimum strength, sliould produce concretions with the most 

 ])erfect symmetry. 



Xo analysis of the inclosing clay has been made, but acid tests indicate 

 a marl-like composition. 



•■** No analysis has been made, imt tieatnienl with IK'l indicates a very high i)er- 

 cenrajie <»f cah;ium carbonate. 



