8 NOTES ON CONCRETIONS 
which is a series of prismatic or vacerous lines See fig. 5, 
enlarged about ten diameters. 
Fie. 5. 
15. On the flat surface of the Potsdam sandstones, Upper 
Cambrian, of the coast of Labrador, fine examples of concretion- 
ary structure may be seen, some of them a yard or more in 
diameter, showing fine concentric lines of various shades of 
color which, when weathered, look like the lines of growth in a 
section of a tree, but no radial lines are seen. The sandstone is 
pierced by Scolithus Canadensis, (Billings). Fig. 6 is from®a 
sketch of a detached piece lying on the shore. 
