162 IF. H. Twenhofel — Silurian Section at Arisaig. 



Dcdmanella cf. edgelliana (sometimes compared with D. subca- 

 rinata), Chonetes tenuistriatus, Spirifer crispus, and Atrypa 

 reticularis. These fossils, and the absence of the guide Clinton 

 or Lower Llandovery fossi^ seem to indicate that Division 

 III is to be correlated with the Rochester (probably within the 

 lower Rochester) and the Upper Llandovery including proba- 

 bly also the Lower Wenlock."* 



Division IVa or Moydart formation. — Basally this divi- 

 sion is on physical characters inseparable from Division III, 

 and on faunal grounds an arbitrary line of separation is drawn 

 where the first Chonetes novascotica appear. The top of Divi- 

 sion IVa is placed at the "Red Stratum," an horizon noted by 

 all geologists studying the Arisaig series. The rocks of this 

 division consist of argillaceous limestones and shales having a 

 united thickness of 379 feet. The guide fossils are, earliest 

 appearance of Chonetes novascotica, suggesting the European 

 C. striatella • Wilsonia wilsoni in typical specimens like 

 those figured by Davidson from the Wenlock ; a rhynchonel- 

 loid suggesting Eatonia medialis, but lias a lamellose instead 

 of a striate surface ; Camarotcechia cf. borealis or formosa / 

 Spirifer subsulcatus ; a later and larger development of S. 

 crispus ; Homoeospira acadice ; Orthoceras suggesting the Lud- 

 low O. striatum : Homalonotus dawsoni / Calymene tubercu- 

 loma / and locally an abundance of ramose bryozoa, but of only 

 one or two species. Faunally, this horizon is intimately con- 

 nected with that of Division III, and as the species compare in 

 development best with those of the middle Niagaran, as the 

 Waldron and Louisville faunas, the time equivalence of Divi- 

 sion I Ya is thought to be of about this time. In North Europe 

 the Wenlock has the nearest faunal approach, and according 

 to Kiaer's work the Moydart fossils agree best with the Upper 

 Wenlock of Norway. 



Division IYb or Stonehonse formation. — This division 

 begins basally at the u Red Stratum,", and is terminated by the 

 amygdaloidal trap. The lower 978 feet consist of argillace- 

 ous limestone and shales, of which about 840 feet have light 

 green to gray colors. A red tinge begins to appear in zone 

 39 and the final 97 feet of zone 40 are of a decidedly brick red 

 color with some green blotches, and shale predominates here 

 over the limestone. 



The fauna is essentially that of Division IYa, but the species 

 have all attained to larger growth and the individuals are 

 j)resent in greater numbers and better preservation. Chonetes 

 novascotica is dominant in fine large specimens throughout 

 this division and in this reminds of the similar development of 

 C. striatella of the Ludlow horizons of Norway (see Kiaer). 

 *See Kiaer, Das Obersilur im Kristianiagebiete, 1908. 



