W. H. Twenhofel — Silurian Section at Arisaig. 149 



been measured in all cases except where the beds are much 

 disturbed or are covered. In the latter cases the thickness has 

 been estimated or calculated. Figures of attitude are mag- 

 netic. 



Collections were made at over two hundred different levels 

 in order to delimit any sudden faunal change to definite hori- 

 zons. The local faunules show that no break in sedimentation 

 occurred and that the disturbances that the rocks have under- 

 gone were not extensive except in a single case at the top of 

 the Clinton. The entire collection reveals a fauna of from 140 

 to 160 species, a total in harmony with Ami's list of 1892. 

 The preliminary identifications of the fossils made by Pro- 

 fessor Schuchert relate only to known forms or to comparisons 

 with well-known species. They are chiefly of brachiopods ; as 

 the equally abundant pelecypods are nearly all undetermined, 

 awaiting future detailed comparison and description. 



Subdivisions of the Arisaig Silurian have been made by four 

 previous writers, as indicated in the appended table, which 

 attempts to show the equivalents of the present section in 

 terms of the older ones. 



Base of Silurian Section. 

 Silurian Division I. 



Zone 1 is usually regarded as the equivalent of the New 

 York Medina, but Professor Schuchert states that all the fos- 

 sils collected belong rather to Clinton time than to the Medina, 

 and that he has found nothing in the collection indicative of 

 the Medina. 



1. Base of section at Beach Hill Cove. Here are exposed 

 greenish calcareous sandstones and thick-bedded argillaceous 

 limestone with some shale. Attitude vertical or even over- 

 turned. Not seen at Arisaig, where the first strata following 

 the break in the shore cliff are banded hornstones. This zone 

 coincides with Honeyman's Division A (1864: 336), with the 

 Medina or Division E 1 of Fletcher (1886 : 37P), and with the 

 lower portion of the Arisaig formation of Ami (1901 : 354). 



Thickness estimated at 160 feet. 



Fossils are scarce and difficult to collect and were found only 

 in the upper seventy-five feet which is exposed at the east and 

 west points of the cove. Lingula cf . oblonga, Dalmanella cf. 

 elegantula, Cornulites flexuosus, and Zajohrentis cf. oila- 

 teralis. 



Clinton Zones. 

 Silurian Division II, or Arisaig Formation. 

 Zones 2 to 12 are correlated with the New York Clinton on 



