no. 5 pre-devonian paleozoic formations 319 



St. Piran Formation p^^^ ^^^^^^ 



I. Massive-bedded quartzitic sandstone 300+ 91-4 + 



Section concealed beneath debris slope. 



MIDDLE CAMBRIAN 



Ogygopsis Shale Member of Stephen Formation 

 This term is applied to the local development of an arena- 

 ceous and calcareous hard gray shale member at the 

 top of the Stephen formation on the northwest slope 

 of Mount Stephen.^ The shale band (lentil) has a 

 maximum thickness of about 150 feet (45.7 m.). It 

 thins out to the northeast and is faulted out to the 

 southwest. At its maximum thickness (2,800 feet, 

 853.4 m-i above Field) it carries immense numbers of 

 trilobites, especially Ogygopsis klotzi (Rominger), 

 Bathyuriscus rotundatus (Rominger), Ncolenus ser- 

 ratus (Rominger), Zacanthoides spinosus (Walcott), 

 and, in addition, sponges, cystids, brachiopods, ptero- 

 pods, and gastropods. The shale is less rich in fos- 

 sils i mile (.4 km.) northeast on the strike; also to 

 the northwest. Lentils of gray quartzitic sandstone 

 and gray siliceous limestone occur in the shale, and the 

 entire shale band appears to be a lentil between the 

 thin-bedded blue limestones and the superjacent, mas- 

 sive, arenaceous Eldon limestone formation. There is 

 no trace of the Ogygopsis shale on Mount Bosworth 

 6 miles (9.7 km.) northeast, at the same horizon, or 

 at Castle Mountain, 20 miles (32.2 km.) east-south- 

 east, but on Mount Field, about 4 miles (6.4 km.) 

 north of the Ogygopsis shale on Mount Stephen, the 

 Burgess shale member of the Stephen occurs in a 

 corresponding stratigraphic position. 

 There is a sharp anticline, with a northeast-southwest axis, 

 in the Ogygopsis shale and the thin-bedded Stephen 

 limestones beneath, on the northwest slope of Mount 

 Stephen (see pis. 71, 72, y^). The southeast limb is 

 crushed and the rocks much altered and cut out by 

 a fault before reaching the amphitheater at the head 

 of Field Brook. On the northwest limb the shales 

 are unaltered and slope down the side of the mountain 

 for 1,800 feet (548.6 m.), thus affording a great ex- 

 posure of the shale and contained fossils. The fauna 

 in the shale includes (14s) : 



Hyolifhellus flagcUutn (Matthew) 



Hyolitheltus annulatus (Matthew) 



Orthotheca corrugata Matthew 



Orthotheca major Walcott 



' Smithsonian Misc. Coll., Vol. 53, No. 5, 1908, p. 210. 



