W. H. Twenhofel — Silurian Section at Arisaig. 155 



Fossils abundant, but poor : Dalmanetla elegantula, Chonetes 

 termistriatus, and an abundance of Cleidophorus. 



26. Gray and greenish-gray argillaceous limestones and 

 slaty nags. ' Steeply upturned. ' Strike N. 46° W. ■; dip 62° VV. 



Thickness 15 feet. 



Fossils as in 24. 



27. Greenish-gray argillaceous-arenaceous limestones in thick 

 beds, many of which are beautifully ripple marked. Strata 

 much disturbed and thrown into a synclinal fold. 



Thickness estimated at 56 feet. 



Division IYa, or Moydart Formation. 



This division is not marked basally by a sudden introduction 

 of new faunal elements and no marked petrologic change 

 occurs, there being a gradual increase in the amount of lime- 

 stone with progress upward. The top of Division IVa is drawn 

 at the upper limit of the " Red Stratum " or zone 32. The 

 total thickness is three hundred and seventy-nine feet. 



The strikingly new and characteristic species is Chonetes 

 novascotica that gradually becomes the dominant fossil of 

 Division IV, attaining to larger and larger size until in Division 

 IVb specimens are more than one inch long on the hinge line. 

 Spirifer crispus of Division III is succeeded in IVa by S. sub- 

 sulcatus and the latter gives rise to S. rugceco&ta that attains 

 to typical development and large size in Division IVb. 



Division IVa forms the upper portion of Honeyman's Divi- 

 sion C (1864 : 336), the upper portion of Fletcher's Niagara 

 or Division E3 (1886 : 38P), and is believed to form the lower 

 portion of the Moydart formation of Ami (1901 : 354). 



28. Greenish-gray more or less arenaceous-argillaceous lime- 

 stone of a prismatic and nodular character, alternating with 

 thick beds of bluish-gray shale that break into irregular frag- 

 ments. Strike N. 80° W. ; dip 30° W. 



Thickness estimated at 117 feet. 



Fossils are scarce but some of the limestone lenses are rich in 

 them. Large crinoid columns, thick branches of a ramose 

 monticuliporoid bryozoan, Dalmanella elegantula, Chonetes 

 novascotica, Camarotcechia cf. formosa or borealis, Spirifer 

 suhsuleatus, Pterinea emacerata, Grammy sia acadica, Ser- 

 pidites cf. dissolutus, Orthoceras sp. 1, Diaphorostoma cf. 

 niagarensis* Homalonotus dawsoni. 



30. Bluish to greenish-gray shales and thin limestones. 

 Beds little disturbed. Strike K 75° W. ; dip 37° W. 



Thickness 32 feet. 



