ORDOVICIAX 345 



number of the species of gastropods and corals that were found in the 

 exposures of dolomite farther up the river. These outcrops were within 

 such short distances of one another and the eastward dip of the rocks is 

 so gentle that it is believed they afford a practically complete section of 

 the Nelson Eiver limestone in this region. 



Sliammattaiva limestone along Nelson River. — The Shammattawa 

 limestone is thought to represent Maquoketa or Eichmond time. Eocks 

 of this age have not been definitely recognized by previous workers in the 

 region. The lowest beds of the Shammattawa limestone are exposed about 

 2^/2 niiles above the Lower Limestone Eapids on Xelson Eiver, about 3 

 miles below the last appearance of the Nelson Eiver limestone, but the 

 contact of the Nelson Eiver and Shammattawa limestones was not found. 

 At the head of the Lower Limestone Eapids a corresponding level of the 

 Shammattawa limestone is exposed, showing a thickness of 7 to 9 feet of 

 gray, thin-bedded limestone overlain by 3 to 5 feet of brown, fine-grained, 

 thin-bedded dolomite. The surfaces of the layers of the dolomite were 

 covered with brachiopod shells and impressions, the more common being 

 Strophomena cf. fluctuosa, Strophomena sp. Rafinesquina cf. alternaia, 

 and Rhynchotrema aff. capax. In the south bluff of the river bordering 

 the Lower Limestone Eapids a ledge of gray, fine-grained limestone, in 

 thick and thin layers, is exposed to a height of about 28 feet. This lime- 

 stone overlies the dolomite horizon last mentioned and contains Rhynclio- 

 trema near capax, Dinorthis aff. sulquadrata, and other fossils character- 

 istic of the Eichmond strata in the LTnited States. 



About 8 miles below the Lower Limestone Eapids a still higher level 

 of the Shammattawa limestone is exposed in a low arch in the south bank 

 of Nelson Eiver to a maximum height of 6 or 7 feet. These strata consist 

 of yellowish gray, rather thin-bedded limestone which contains fragments 

 of Isotelus and other species of fossils characteristic of the deposits of 

 Eichmond time. 



Poi't Nelson limestone along Nelson River. — The Port Nelson lime- 

 stone is the oldest Silurian formation known in the Hudson Bay region. 

 It consists of yellowish brown, rather fine-grained dolomite, certain layers 

 of which contain casts and molds of shells of Virgiana decussata in great 

 numbers. The thickness is probably somewhat more than 30 feet. As 

 far as known, this limestone outcrops nearer to Port Nelson, at the mouth 

 of Nelson Eiver, than any other formation of Paleozoic age ; hence it is 

 designated the Port Nelson limestone. 



The succession and the stratigraphic and faunal relations of the Paleo- 

 zoic rocks exposed along Nelson Eiver are shown in the detailed section 

 and fossil lists given below. The species m^arked sp. in the fossil lists on 



