is to be looked for in the scattered shells and other ealcareons 

 remains, which were dissolved hv the pereolatins^ \vaters. 

 The exterior of the concretion seldom shows any veining, 

 but when broken, a series of calcite veins, usually branchinu- 

 and intercrossing, is seen. These veins are often beautifullv 

 banded, exhibiting white crystalline calcite in the center, and 

 successive bands of darker impure calcite towards the margin. 

 The veins are largest in the center and thin out towards the 

 periphery of the concretion. When exposed to the mechanical 

 wear of the stream, and to the solvent action of the water, 

 the oiitcr crust is removed, and as more and more of the 

 clavstone is worn away the veins begin to stand out in 

 relief, bccaiise the pure crystalline calcite is much less solvd)lc 

 than the amorphous particles which cement the clay. The 

 s-eptate or divided appearance thus produced has given rise 

 to the name "septaria," commonly applied to this class of 

 concretions. Where a considerable portion of the concretion 

 has been worn awa}-, the calcite veins — usually stained 

 yellow or brown by hydrous oxide of iron — appear very 

 prominent, and by their intercrossing cause a resemblance of 

 the concretion to the back of a turtle, on which account 

 these rocks are often called "turtle stones," "turtle backs" 

 or "petrified turtles." 



Large numbers of these concretions, derived from the shale 



banks above the bridge, are carried down every spring by 



the floating ice, and strewn over the flats and the river bed 



in the lower portion of the gorge, where they form one of the 



curious attractions, exciting commonly more interest than 



the large numbers of finely preserved fossils occuring with 



them. 



Rei;arc'iiii<i; the- mode t)t" formation ot" these concretions little is known. 

 They iindonbtedly bear a genetic relation to the clay stones common in 

 tinconsolidated deposits in various portions of this and other countries. 

 When lithification of the concretion begins, chiefly through the loss of 

 the combined water, a radial contraction takes place, which must be 

 towards the ]icriphery of the concretion, since the weight of the super- 

 incumbent strata prevents the formation of cracks in the outer shell. 

 Conscciuently the cracks are widest towards the center and disappear 



