302 THE CANADIAN NATURALIST. [Vol. il. 



THE HELDERBERG ROCKS OF ST. HELEN'S ISLAND. 



By J. T.Donald, B. A., 



Science Master, High School, Montreal. 



The second great limestone formation in the Upper Silurian 

 series of rocks has received the name Helderberg from its 

 occurrence in the mountains of this name in the State of New 

 York. It is found in several localities in eastern New York, in 

 Gaspe, and also in various parts of New Brunswick and Nova 

 Scotia, as well as western Ohio and Indiana. 



Between these extreme limits we have but one isolated patch of 

 genuine Helderberg rocks, and that is to be found on St. Helen's 

 Island, in the St. Lawrence opposite Montreal. This island is 

 almost entirely made up of a dolomitic conglomerate or volcanic 

 breccia, in all probability poured forth from the ancient volcano 

 of which Montreal Mountain is but the base. Associated with 

 this breccia are certain masses of hard gray limestone, of Helder- 

 berg age. The existence of this limestone was well known for 

 a long time before its geological age was recognised. Occupying 

 the position it does near the Trenton of Montreal Island, it was 

 naturally thought to belong to this formation. To Dr. Dawson 

 is due the credit of having discovered that it was of Helderberg 

 age. Being out, as is his custom, with a party of students, he 

 broke off a fragment of the rock, expecting to find Trenton 

 fossils if any. To his surprise, he was confronted with Helder- 

 berg species. 



Rocks of Helderberg age must have been extensively deposited 

 over the area reaching from Gaspe, New Brunswick and New 

 York to Ohio and Indiana, and then in great part removed by 

 denudation. The masses on St. Helen's Island have resisted 

 this denuding action, being protected by the hard breccia which 

 encloses them. This deposit of Helderberg limestone is of in- 

 terest as being the only representation of this formation between 

 the limits before indicated and, further, because it is distant 

 nearly 200 miles from the nearest position of the group elsewhere. 

 I have said this deposit is the only genuine representation of 

 Helderberg within the limits mentioned, for, although we find 

 elsewhere, as at Isle Bizard, Riviere-des-Prairies and Ste. Anne, 

 rocks which are called Helderberg, they are so called on the 



