Formation of St. Helen's, Island. 105 



visible at the upper extremity of the Island. The deposit 

 consists principally of a conglomerate, the enclosed masses 

 of which are sometimes rounded, but chiefly angular. They 

 consist of fragments of Laurentian gneiss ; of white 

 quartzose sandstone resembling that of the Potsdam forma- 

 tion ; of dark grey limestone in some cases holding Trenton 

 fossils ; of black shale resembling that of the Utica forma- 

 tion ; and of red sandstone and red shale similar to those of 

 the Medina. With these fragments are associated others of 

 igneous rocks. All of these, varying in size from a quarter 

 of an inch to five and six inches, are enclosed in a paste of 

 a light grey dolomite, which weathers to a reddish-yellow." 

 " About two-thirds of the distance down the east 

 side of the Island, there occur two masses of dark-grey 

 fossiliferous limestone, weathering to a light grey. . . . 

 They have a breadth of scarcely more than ten feet, and 

 appear to run under the dolomitic conglomerate. 

 The fossils observed in the limestone are Favosites Goth- 

 landica, Strophomena rhomboidalis, S. punctulifera, Orthis 

 oblata, an undetermined species of Rhynchonella with R. 

 Wilsoni, Athyris bella, Atrypa reticularis, and two undeter- 

 mined species of Spiri/era." 



Exposures indicating the same kind of agglomerate were 

 discovered at Eound Island, Isle Bizard, Riviere des Prairies, 

 and near Ste. Anne, but no limestone. 



In addition to the localities mentioned a small patch of 

 similar agglomerate has been found on the McGrill Grounds, 

 immediately behind the Medical building, and probably 

 many others may eventually be found on the Island of 

 Montreal. 



Sir William Dawson has in papers, and addresses to the 

 Natural History Society of Montreal, stated reasons for 

 regarding the agglomerate as a portion of the fragmental 

 ejecta of the old Silurian volcano of Montreal, which con- 

 tinued active up to and beyond the time of the deposition of 

 the Lower Helderberg limestone. Some of these trappean 

 dykes are found cutting and altering the limestone. This 

 conclusion is supported by the angular character of the 



