16 INDEX TO THE STRATIGKAPHY OF NORTH AMERICA. 



POST-CAMBRIAN INTRUSIVE ROCKS. 



Color, dark red. 



Symbol, 24a. 



Distribution: The Cordillera from South America to Alaska; Santo Domingo; eastern United 



States, New Brunswick, Nova Scotia, and Newfoundland. 

 Content: Plutonic rocks of all kinds, chiefly granites, occurring in masses of sufiicient size to be 



mapped. Mostly -of Paleozoic and Mesozoic age. Includes some effusive rocks of same 



age and some older intrusives. Tertiary intrusives mapped with "Tertiary and later 



effusive rocks." 



TERTIARY AND LATER EFFUSIVE BOCKS. , 



Color, scarlet. 



Symbol, 25. 



Distribution: Volcanic rocks and lava flows, chiefly of Tertiary age, of the Cordillera from Cen- 

 tral America to Aleutian Islands; Venezuela and Windward Islands; Greenland and 

 Iceland. Includes some intrusive rocks of same age. 



In applying the preceding legend to the map adjustments to scale and to 

 knowledge have been made by the compilers, but adjustments of classifications 

 adopted by different surveys could be made only by authority of the responsible 

 officers, and to bring them about has necessitated correspondence or required special 

 conferences. The classification of the pre-Cambrian was discussed at two formal 

 conferences at which the Canadian and Federal surveys were represented, with 

 results that are stated in Chapter II. 



Some other difficult questions may be enumerated here. 



In applying the distinction indicated by the categories 23b and 23c, the com- 

 pilers have placed in the former all areas where Cambrian sediments rest uncon- 

 formably upon the pre-Cambrian. In 23c are classed those areas which contain 

 highly metamorphic rocks overlain by later Paleozoic or Mesozoic strata and which 

 may include masses that were deposited and metamorphosed in some post-Cambrian 

 epoch. This is the case throughout the Cordillera, except ia the southwestern 

 United States. 



The "Archean" of the southern Appalachian Mountains has been separated 

 by Keith into undifferentiated pre-Cambrian, pre-Cambrian intrusives, and post- 

 Cambrian intrusives. The undifferentiated pre-Cambrian is carried northward 

 through New Jersey, New York, and New England to the international boundary. 

 In the Adirondacks this division presumably comprises the "Laurentian" of 

 Canada and also the Hastings and Grenville representatives. 



The pre-Cambrian rocks of Sutton Mountain, Quebec, are mapped as igneous 

 and undifferentiated pre-Cambrian, after Dresser. The distinction can not yet be 

 carried across the international boimdary. 



The Random formation of Newfoundland is placed in the late pre-Cambrian 

 by Walcott, and the rest of the Avalon group is mapped with it. 



The gold-bearing Meguma series of Nova Scotia is classed as pre-Cambrian 

 on the recommendation of C. K. Leith and because of the resemblance which the 

 rocks bear to the Avalon group of Newfoundland, as noted by Selwyn, Murray 

 Howley, and Faribault. 



The Belt series, though regarded as Cambrian by R. A. Daly, is retained in 

 the Algonkian in accordance with the observations of Willis, confirmed by the later 

 work of Walcott. 



