200 



Grout and Broderick — The Huronian. 



conglomerate bed is a second zone, a few feet thick, of 

 cherts with the structures of much smaller size, supposed 

 to be organic. In nearly every outcrop of the chert, a 

 bed several inches thick can be followed for several 

 yards, with the organic structures as abundant as they 

 are shown in figs. 2 and 3. There are commonly finger- 

 like masses, such as are shown by Leith's plate. 4 Above 

 the conglomerate is thin-bedded taconite to the top of the 

 formation. The two beds, one near the base of the for- 



FlG. 1. 



Fig. 1. Skeleton photograph of a vertical section of lower algal beds; 

 Biwabik formation. 



mation, and one near the top of the cherty conglomerate, 

 are remarkably persistent, and show an abundance of the 

 organic structures throughout. The chert is hard and 

 outcrops in many places. 



Mineral Composition of the Organic Forms. 



These rocks showing structures attributed to organ- 

 isms are made up of ferruginous chert. Most of the drill 



4 Leith's description says the forms are characteristic of the basal hori- 

 zon, but the basal beds are usually much coarser than the one illustrated, 

 whereas the upper bed is in most places much like the specimen he shows. 



