256 ROYAL SOCIETY OF CANADA 



ten miles away, at several places on He Bizard, and on the slopes of the 

 easternmost of the Oka mountains at La Trappe, still farther to the west, 

 A small breccia dyke is also found near Ste. Anne de Bellevue. 



The Ste. Helen's Island and He Eonde bodies, having an area of 

 about seventy and thirty acres respectively, are by far the largest of the 

 various occurrences, the next in size being only of the order of five acres. 

 The large mass at La Trappe has relatively few fragments or inclusions 

 and should not strictly be included here. At first sight these occur- 

 rences of breccia do not seem to be of any particular importance, nor to 

 have any especial connection with one another, but on an inspection of 

 the map it is found that they also are related to a certain definite narrow 

 belt and that this belt is a continuation of the line of the Monteregian 

 Hills, adding to it an extra length of twenty miles. 



Descriptions of the Various Occurrences of Breccia. 

 (In order from west to east.) 



Near La Trappe. 



On the southeastern fringe of the group of Laurentian hills forming 

 what are known as the Oka Mountains, there are three occurrences of 

 breccia. About one mile east of the Trappist Monastery (known as La 

 Trappe), a mass of breccia is found intruding the Laurentian gneiss and 

 the Potsdam sandstone. Immediately to the west of the Monastery there 

 is a hill formed of an intrusive with a brecciated border zone. Again to 

 the west of this hill there is a large d^'^ke of breccia. 



The first of these occurrences has an elongated outline, about 

 seventy-five paces wide and traceable for two hundred and seventy-five 

 paces, in which distance it has narrowed doTVTi to a dyke a few feet wide. 

 The narrow extension lies parallel with the schistosity of the gneiss. The 

 mass seems to be due to a dyke which has been very much widened in 

 one portion. 



The second occurrence forms a hill about one hundred feet high 

 and a quarter mile across. It is caused by an intrusive plug which has 

 pierced the Grenville Limestone and Laurentian gneiss near the contact 

 of the two. The main mass is almost pure intrusive material, but the 

 border zone is a breccia formed from the gneiss, limestone, and Potsdam 

 sandstone. The Grenville is shot through with stringers of the igneous 

 material and there are several large offshoots running away from the hill 

 on the north side. (See fig. 1.) Several dykes of a camptonite like rock 

 cut these outliers where they cross the highway. An interesting example 

 of a pseudo-miarolitic cavity was observed in the igneous portion. It was 

 caused by a fragment of the Grenville limestone having been partly ab- 



