250 Williams — Geology of Arisaig-Antigonish District. 



Through the surrounding rocks there are many small rhyo- 

 lite dikes which appear to be connected with the general 

 rhyolite intrusion. An aporhyolile or devitrified rhyolite flow 

 probably 200 feet thick rests at the base of the Silurian section 

 and is of historical interest because it was long mistaken for 

 metamorphosed sediments on account of its banded structure. 

 " Eozoon " forms were at one time reported from it. 



None of the above intrusives is known to cut any rocks 

 younger than those of Upper Cambrian age, and excepting the 

 monzonite all are perhaps of related origin. Fragments of 

 rhyolite are common in the Malignant Cove conglomerate, 

 which is supposed to be of early Ordovician age, and fragments 

 representing the James JRlver granite and the monzonite are 

 also thought to have been recognized in the conglomerate 

 deposits. The available evidence thus favors the supposition 

 that the intrusion of rhyolite, granite, quartz porphyry, and 

 monzonite, and the extrusion of the aporhyolite flow took place 

 during late Cambrian or early Ordovician time. Obscure 

 tuff beds associated with the Upper Cambrian iron-ore zone 

 indicate volcanic activity at a still earlier date, but the deposits 

 are too much altered to shed much light upon the characters of 

 such remote eruptions. 



At a number of localities in the district diabase occurs either 

 as irregular necks, which is the case at the Sugar Loaf Hill 

 north of Antigonish town, or else as dikes, generally but a few 

 feet across. In a number of cases the basic intrusives are of a 

 basaltic rather than of a diabase nature. Unlike the igneous 

 rocks already described, the diabase cuts rocks of all ages from 

 Upper Cambrian to early Mississippian. Apparently the intru- 

 sion took place during one general activity, and so the diabase 

 dikes and necks are probably of early Mississippian age. 



Intimately associated with the diabase intrusives along the 

 shore east of Arisaig Point is a long red dike of soft fissile 

 character. It cuts the diabase dikes and the aporhyolite flow. 

 This dike has been traced for nearly 3 miles, although numer- 

 ous breaks occur in it, and part of the way two dikes are pres- 

 ent instead of one. Studied microscopically, the red dike is 

 seen to contain much iron oxide, but it evidently was originally 

 composed of clastic material. It is thought to have been of a 

 fine breccia nature, which may have originated during pulsations 

 of material which reached nearly to the surface. In places the 

 red dike is associated with basalt which it cuts. It is possible 

 that the basalt and the red dike represent late phases of the 

 diabase intrusion and are essentially of the same age. 



Thus igneous activity represented in the Arisaig-Antigonish 

 district is thought to have been confined for the most part to 

 late Cambrian or early Ordovician time, and to the early part 

 of the Mississippian period. 



