352 R. W. ELLS — LAURENTIAN OF THE OTTOWA DISTRICT. 



several corrugated bauds of gneiss are included, the whole thickness of 

 the limestone representing less than forty-six feet, and the greatest 

 development of the calcareous members being near the top of the section. 

 The gneiss is of several kinds, but mostly reddish, grayish, hornblendic 

 or rusty. From the character of the section, it is evident that the greater 

 part lies below the calcareous portion of the system or forms the lowest 

 part of that division. 



Origin of the Term Laurentian. 



The term Laurentian, as applied to the lowest system in Canadian 

 geology, first appears in the report for .1852-'53. It was founded on the 

 name Laurentian given by Mr Garneau, of Quebec, to the range of hills 

 on the north side of the Saint Lawrence river, which are composed prin- 

 cipally of the rocks of this system. In this report the description of the 

 country west of the Ottawa and north of Kingston on the Saint Law- 

 rence is by Mr A. Murray, and therein he describes a similar series of 

 grayish and reddish gneisses with crystalline limestone. The latter, 

 however, has a much greater development than in the section on the 

 upper Ottawa. Intrusive masses of red granite are noted at various 

 points, which cut transversely across the gneiss. The structure of the 

 limestone in places is held to be interstratified with the gneiss, but at 

 other points it appears on either side of an anticlinal in the gneissic 

 rocks. 



Sir W. Logan's Investigation of the Laurentian Structure. 



The principal work on which the structure of the Laurentian has been 

 based for many years was done by Sir William Logan and his assistant in 

 1853. in the Grenville district, north of the Ottawa river. This area is 

 situated about midway between Ottawa and Montreal, and the diffi- 

 culties he encountered in the attempt to unravel what has long proved a 

 puzzling problem were very great. The country at that time was almost 

 entirely a wilderness, small sections only being opened up for settle- 

 ment, densely wooded and, in places, largely drift-covered. Short trav- 

 erses were made along some of the larger lakes and on the principal 

 streams or by means of tracks cut through the forest. Of the difficulties 

 as to the structure, Sir William says : 



" Bands of crystalline limestone are easily distinguished from bands of gneiss, 



but it is scarcely possible to know, from mere local inspection, whether any mass 

 of limestone in one pari is equivalent to a certain mass in another. They all re- 

 semble one another more or less Lithologirally, ami although masses are met with, 

 running for considerable distances rudely parallel to one another, it is not yet 



