156 W. H. Twenhofel — Silurian Section at Arisaig. 



Fossils abundant : Chonetes novascotica, Camarotcechia cf. 

 formosa, Hommospira cf. acadice, H. cf. evax, Spirifer sub- 

 sidcatus, Orthoceras sp. 1, 0. sp. 2, Homalonotus dawsoni. 



31. Greenish-gray more or less heavy-bedded argillaceous 

 limestones, some beds of nearly pure limestone, bluish-gray 

 mudstones cleaving into thin beds, soft shales, and splintery 

 arenaceous flags. The zone begins just east of the mouth of 

 McDonald's brook. Strike JSL 72° W. ; dip 36° W. 



Thickness 101 feet. 



Ramose bryozoa as in 28 ; Camarotoechia cf . formosa / an 

 Eatonia medialis-\ike rhynchonelloid, but not at all this 

 species; Spirifer subsulcatus • Homoeospira cf. acadiae / 

 Orthoceras sp. 2 ; Calymene tuberculata / Homalonotus davj- 

 soni ; Cornulites proprius. 



32. The " Red Stratum." A brick-red shale of which the 

 upper thirty feet is prismatic and locally nodular. Shows 

 little evidence of stratification except near its base. It is rather 

 sharply differentiated from the overlying green shales, but 

 grades into the subjacent zone. Twenty feet below the top is 

 a nodular band ten inches thick. The nodules are bright green 

 to greenish-white in color and have their longer axes trans- 

 verse to the bedding. The same color shows along the fracture 

 lines. At the base are included twenty-seven inches of thin 

 beds of ferruginous limestone and shale which form the tran- 

 sition to zone 31. Strike K 68° W. ; dip 40° W. 



Thickness _ . _ 32 feet. 



Division IVb, or Stonehouse Formation. 



This subdivision has its base on the top of the " Red Stra- 

 tum " and extends to the amygdaloidal trap at the top of the 

 Silurian section. It is faunally characterized by the large size 

 of the species and especially by the abundance of Pholidops 

 implicata, Chonetes novascotica, Spirifer rugwcosta, Ilomoeo- 

 spira, n. sp. 1, Grammy sia acadica, G. rustica, Pteronitella 

 ve/msta, P. curta, Calymene tuberculata, Acaste logani, 

 Homalonotus dawsoni, and an abundance of Beyrichia 

 pustidosa and B. wquilatera. The subdivision has a total 

 thickness of one thousand and seventy-five feet. It corre- 

 sponds to the upper portion of the Moydart formation of Ami 

 and the whole of his Stonehouse formation (1901:354). It 

 coincides with Division D of Honey man, as described in the 

 Quarterly Journal of the Geological Society of London (1864 : 

 336), and to the Lower Helderberg or Division E6 of Fletcher 

 (1886 : 37P). 





