112 THE JOURNAL OF GEOLOGY. 



are not uncommon, and hornblendic and pyroxenic gneiss appears in some 

 places. The Fundamental Gneiss, so far as at present known, is a compli- 

 cated series of rocks, for the most part of unknown origin, but comprising a 

 considerable amount of intrusive material. 



In certain parts of the Laurentian area, and notably in the Grenville dis- 

 trict, the Laurentian has a different character. In the Grenville series the 

 orthoclase-gneiss is still the predominating rock, although it here has a greater 

 variety of mineralogical condition, and is frequently well foliated. Amphibo- 

 lites, hornblende-schists, heavy beds of quartzite, and numerous thick bands 

 of crystalline limestones, are all abundant and interstratified with one another. 

 In the series are ores, and a wide variety of minerals. In the limestone and 

 associated rocks graphite is often widely disseminated. This does not occur 

 in the Fundamental Gneiss. The areas occupied by the Grenville series 

 while together aggregating many thousands of square miles, are probably 

 small as compared with those of the Fundamental Gneiss. The Gren- 

 ville rocks, while generally highly inclined, over some large area are 

 nearly horizontal, but even in these cases they have been subjected to great 

 pressure. 



As to the origin and relations of the Fundamental Gneiss and the Grenville 

 series, three views may be taken : 



1. The Fundamental Gneiss may be the remains of a primitive crust pen- 

 etrated by great masses of igneous rocks and having been subjected to 

 repeated dynamic movements. The Grenville series may be an upward con- 

 tinuation of the Fundamental Gneiss under altered conditions, marking a 

 transition from a primitive crust to normal sediments. Thus the two would 

 form one practically continuous series. The general petrographical similar- 

 ity of the two series, taken in connection with the more varied nature of the 

 Grenville series, its frequent stratified character, and the presence in it of 

 limestones and graphite indicating an approach to modern conditions and the 

 advent of life, together with the difficulty of clearly separating the two series 

 from each other and defining their respective limits, lend support to this 

 view. 



2. The Grenville series may be considered as distinct from the Funda- 

 mental Gneiss, and reposing on it uncomformably, being a highly altered 

 series of clastic origin, the Fundamental Gneiss having some such origin as 

 suggested above or being an older series of still more highly altered sedi- 

 ments. As it is now thoroughly crystalline, there is, however, no absolutely 

 conclusive proof that even the Grenville series is of sedimentary origin. How- 

 ever, the series is in all probability made up, in part at least, and perhaps 

 wholly, of sedimentary material, but as this is not absolutely shown, the pro- 

 posal to separate it from the rest of the Laurentian and class it as Algonkian 

 or Huronian seems premature. 



