316 THE CANADIAN NATURALIST. [Vol. viii. 



Professor Bell, of the Geological Survey, has repeatedly noticed 

 the occurrence of dykes in the regions explored by him north of 

 Lakes Superior and Huron, and states that in some parts of the 

 country they form a conspicuous feature in the geology, and have 

 probably played an important part in producing the present 

 geographical features. One described by him as a diorite in 

 the report of the Survey for 1875-76 (p. 314) is said to be from 

 300 to 400 feet in width. Its course is N. 12° W., and it cuts 

 a thinly bedded micaceous gneiss nearly at right angles to the 

 strike of the latter. 



Mr. G. M. Dawson has also given us a number of facts con- 

 cerning dykes at the Lake of the Woods, where they are said to 

 be both granitic and dioritic. Some of the latter, which are 

 coarse-grained and apparently have general east and west courses, 

 "may very probably be among the oldest of the intrusions." 

 There are others, however, which are very hard and compact, 

 and have a general bearing of north-east and south-west. These 

 cut not only the intrusive granites of the region, but also the 

 altered Laurentian strata.* 



The late Mr. Walter McOuat has mentioned the occurrence 

 of dykes of " diorite " from fifty to one hundred feet thick at 

 several localities between lakes Temiscamang and Abbitibbe, and 

 states that the apparent direction of two large ones on Lac des 

 Quinze (on the Upper Ottawa) is north-by-east and south-by- 

 west, f 



It is therefore evident that in almost all parts of the country 

 where the Laurentian rocks have been examined, they have been 

 found to be cut by dykes of various intrusive rocks, few of which 

 have, however, been critically studied as yet. 



The intrusive rocks of the Grenville region are of special inte- 

 rest, inasmuch as most of them were shown by Sir William Logan 

 to belong to a date anterior to. the deposition of the Lower Sil- 

 urian. According to the descriptions given in the Geology of 

 Canada, they consist of dolerite, syenite and felsite porphyry. 

 Of these the oldest " are a set of dykes of a rather fine-grained 

 dark greenish-grey greenstone or dolerite, which weathers grey- 

 ish white." * * * " Their width varies from a few feet to 



* See Report on the Geology and Resources of the Region in the 

 vicinity of the Forty-ninth Parallel. 1875. pp. 25, 53. 



f Report of Progress, Geol. Survey, 1872-73, pp. 120, 122 and 130. 



