64 Canadian Record of Science. 



eroded surface E. F. Later accumulations would build 

 the cone up to the line G-. H. After the volcanic 

 action had ceased, erosion would reduce the land to the 

 line A. B., leaving isolated patches of the base of the cone 

 or basal breccia. Of the explanations, the latter seems 

 to be the more probable. 



The breccia is post-Oriskany in age and may possibly 

 be later Devonian. 



Volcanic activity continued after its deposition as 

 evidenced by the many dykes which cut it. 



A point to be noted in this connection is that the 

 Alnoite dykes must be older than the series about Mount 

 Eoyal for as mentioned in the descriptions of the He 

 Bizard and White Horse Eapids breccia Alnoite frag- 

 ments are frequently found in those occurrences. 



Petrography of the Dyke Rocks. It was stated on a 

 former page that the Utica shale and the breccia on St. 

 Helen's Island were cut by a series of dykes. About 20 

 of these were noted and mapped (see fig. 1), but it is 

 probable that many more are concealed by the drift. 

 They vary in direction from N. 60° W. to N. 80 Q W. 

 Their width varies from 1 to 6 feet. Their length could 

 not be ascertained as the drift on the one hand and the 

 river on the other cut them off. 



In addition to the dykes, a prominent sill occurs at the 

 south end of the island. It is in two beds 0.8 and 1.6 feet 

 respectively with a thin layer of altered shale between. 

 Both the dykes and the sills belong to the great series 

 which cut the Ordovician formations in this district, and 

 which represent a phase of the Mount Royal eruption. It 

 was found by the aid of the micoscope that these rocks 

 could be divided into two classes, one containing feldspar, 

 while in the other the mineral is absent. 



Of the feldspathic class a specimen from the upper bed 

 of the large sill was taken as a type. 



Macroscopically the rock is porphyrinic, dark grey in 



