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



Stonehouse brook. Very extensively veined by quartz and 

 calcite. Strike K 52° W. ; dip 43° W. 



Thickness 136 feet. 



Many fossils : Pholidops implicata, Chonetes novascotica, 

 Camarotoechia cf. nucula, 0. cf. horealis, Spirifer rugcecosta, 

 Homwospira n. sp. 1, Cornulites proprius (?), many bivalves, 

 Beyricliia wquilatera, B. pustulosa, Acaste logani, Calymene 

 tubercidata . Ilomalonotus dawsoni, Eurypterus or Pterygotus 

 fragment. 



40. Red shales and limestones with grayish-blue splintery flags 

 dotted with very bright green patches. ' In the flags the fossils 

 occur in thin calcareous layers attached to their under sides. 

 Strike 1ST. 63° W. ; dip 30° W. 



Thickness 97 feet. 



Has in addition to the fossils of 39 the following : Schuchertella 

 suhpjlana, Pteronitella venusta, Bucanella trilobata (large), 

 Grammy sia acadica, Goniophora transiens. 



41. Amygdaloidal trap overlain unconformable by Carbon- 

 iferous (?) sandstone. Apparently has altered neither the 

 Silurian strata nor the sandstone, but Silurian contact obscure. 

 Strike of upper contact, K. 32° E. : dip 23° E. 



Estimated thickness 40 to 50 feet. 



Volcanic Rocks at the Base of the Silueian. 



On a previous page mention has been made of the rocks 

 underlying the Silurian strata and it was stated that these rocks 

 have been for the most part considered as altered sedimenta- 

 ries. For complete details regarding them the reader is referred 

 to the Report of the Canadian Geological Survey for the year 

 1886, page 9P. Studies made of these rocks in the field led 

 the writer to the conclusion that practically all are of volcanic 

 origin, which seems also to have been the view of Fletcher 

 (1886 : 9P). Since, however, they have so often been referred 

 to as altered sedimentaries, it seemed desirable to investigate 

 them by chemical and petrological means, and with this pur- 

 pose in view specimens were collected from Arisaig Pier to 

 Frenchman's Barn. These specimens have been studied by 

 the writer in the Petrological Laboratory of the Sheffield 

 Scientific School under the supervision of Professor L. V. 

 Pirsson and in the Kent Chemical Laboratory under the direc- 

 tion of Professor F. A. Gooch ; to each of these gentlemen 

 the writer acknowledges his indebtedness. 



The rocks, as first seen at Arisaig Pier, consist of relatively 

 light-colored hornstones which show banding to a high degree. 

 Eastward thev become coarser and darker and at the western 



