Geology of St. Helen's Island, 55 



are genetically connected with the Mount Eoyal intrusion. 

 These will be referred to more in detail later on. 



Pleistocene. The drift covering extends over al- 

 most the whole island. It varies in thickness from 

 practically nothing on the breccia hills to 35 feet along 

 part of the western shore, where the St. Lawrence Eiver 

 has cut a terrace in it, which reveals the whole section. 

 In composition, the drift is an unmodified sandy clay, 

 holding large and small glaciated boulders, principally of 

 Laurentian and Trenton age, though the intermediate 

 formations are also represented. The augen, syenitic and 

 garnetiferous gneisses are particularly noticeable. Towards 

 the top of the deposit fragments of breccia are common, 

 as well as a few erratics from Mount Royal. 



Ordovician. The Utica formation is only exposed at 

 the south end of the island, the rest of the area underlain 

 by it being covered by the drift. The rock is a dark 

 almost black highly bituminous shale, weathering in 

 places to a rusty brown. The ease with which it disin- 

 tegrates into small thin laminae, has masked both the 

 strike and dip. However it is probable that the beds 

 are practically horizontal, or dip to the east at a very 

 low angle. No fossils were found though a diligent search 

 was made. When cut by dykes and sills, a narrow zone 

 of shale in contact with the igneous rock has been altered 

 to hornstone. With one exception, the contact between 

 the breccia and shale is concealed by drift and the ap- 

 proximate boundary, (see map, fig. 1), which may be con- 

 sidered fairly accurate, was therefore drawn from the evi- 

 dence furnished by the topographic features. 



Devonian. (?) The breccia, which underlies the remainder 

 of the island, is an unstratified massive rock. It is com- 

 posed of fragments of rocks, which are angular, subangular^ 

 or rounded with facets, but not waterworn. These frag- 

 ments vary in size from microscopic grains to boulders 12 

 and 15 inches in diameter, and the range in age extends 



