Grout and Broderick — The Huroniau. 2<>3 



appearance of the lower cherts is shown in figure 1, 

 in which the lines of a photograph are emphasized by 

 drawing. The structures differ markedly from algal 

 forms as described from some other formations, in not 

 being concentric and hemispherical. They show a 

 marked tendency to develop columns, with a structure 

 like a pile of similar inverted bowls. The higher bowls 

 may be larger or smaller than the lower, but probably 

 start at the bottom of the pile with a small dome and con- 

 centric structures. It seems likely that many small 

 centers of growth developed hemispherical forms till 

 they interfered with each other, and then developed by 

 successive layers on the top, with more or less interfer- 

 ence laterally from their neighbors. This interference 

 may in the average case practically merge the layers of 

 adjoining columns into a single undulating layer, gently 

 convex on the domes, and sharply concave between. The 

 columns range from two inches to two feet in diameter. 

 At certain places on the undulating surface new centers 

 may start and interrupt the regularity of the columns 

 and bands. 



The upper zone shows no such coarse structure and 

 should apparently be attributed to a different species ; in 

 detail it shows much clearer, more distinct columns. 

 Resting between thick beds of highly ferruginous intra- 

 formational conglomerate, the bed of chert is in sharp 

 contrast with its surroundings. However, near the 

 cherty beds the conglomerate contains numerous frag- 

 ments of chert, and it is evident that the organic growth, 

 though extending continuously many miles, does not indi- 

 cate any radical change of conditions from those of con- 

 glomerate formation. 



In the best exposures, the organic forms resemble 

 finger-like columns, with a structure that may be 

 described as that of a pile of thimbles. In diameter the 

 columns are about like fingers and may be scattered an 

 inch or more apart, though usually much closer. They 

 stand much more distinct from their neighbors than the 

 larger columns of the lower horizon. The spaces around 

 each column are filled with granular matter and pebbles 

 exactly like those of the conglomerate. The bands of 

 iron oxide in chert, which mark the conspicuous struc- 

 ture, do not run out in most cases anions the fragmental 

 grains; but enough cases have been discovered where 

 they do run among the pebbles, and even around them 



