6i8 FRANK D. ADAMS 



The rocks which compose the Laurentian Mountains were shown by the 

 Geological Survey in 1846 to consist of a series of metamorphic sedimentary 

 strata underlying the fossiliferous rocks of the Province. They have since been 

 recognized by Sir Roderick Murchison as forming the so-called "Fundamental 

 Gneiss" of the western islands of Scotland and parts of Rosshire and Suther- 

 landshire, and the name of the Laurentian system as applied in Canada has now 

 been extended to them in Great Britain where, as well as in this country, they 

 are the oldest rocks known and lie at the base of the sedimentary series. They 

 are highly altered to a crystalline condition and are composed of feldspathic 

 rocks interstratified with important masses of limestone and quartzite. The 

 great vertical thicknesses of the series are composed of gneiss containing chiefly 

 orthoclase or potash feldspar, while other great portions are destitute of quartz 

 and composed chiefly of a lime soda feldspar, varying in composition from ande- 

 sine to anorthite and associated with pyroxene or hypersthene. This rock we 

 shall distinguish by the name of anorthosite. 



Logan's exploratory work showed that these ancient rocks under- 

 lie enormous areas in Canada. It is now known that they occupy 

 an area of about two million square miles. As their structure is 

 very complicated, he found it impossible to make a detailed study of 

 the whole Canadian shield and therefore decided to select a com- 

 paratively limited area in which the Laurentian system had a typical 

 development, and there, by careful mapping, to ascertain its structure 

 and relations. The district which he selected had an area of about 

 fifteen hundred square miles and was situated on the margin of the 

 northern protaxis in the vicinity of the little town of Grenville, some 

 forty miles to the west of the city of Montreal. His map of this area 

 appeared in the atlas which accompained The Geology of Canada. 

 This area has come to be known as the "Original Laurentian Area" 

 of Logan, and the succession which he worked out there has found 

 its way into most textbooks. 



Since that time other members of the staff of the Geological Survey 

 of Canada, notably Vennor and Ells, have extended our knowledge 

 of this portion of the Laurentian area by mapping large tracts along 

 the margin of the protaxis to the west of the Original Laurentian Area 

 as far as the western border of the Province of Quebec, and thence 

 over into eastern Ontario. In 1897' the writer published the results 



I "Report on the Geology of a Portion of the Laurentian Area lying to the 

 North of the Island of Montreal," with appendices and map, Annual Report of the 

 Geological Survey of Canada, Vol. VIII, p. 184. 



