GRENVILLE SERIES OF EASTERN NORTH AMERICA 621 



lying between and below the limestones, as well as that exhibited by 

 the anorthosite, was regarded as the survival of an almost obliterated 

 bedding, and this foliation, as has been mentioned, extended down 

 to the very base of the whole series. The existence of this great thick- 

 ness of anorthosite superimposed upon, and, therefore, presumably 

 younger than the orthoclase gneiss, was in a brilliant paper by Sterry 

 Hunt, explained as due to a succession of chemical reactions which 

 must necessarily have been developed during the cooling of the earth 

 from whose primeval ocean the whole series was supposed to have 

 been deposited in the order of their succession.^ 



Since Logan's time, however, the great advance in our knowledge 

 of petrography has thrown a flood of light upon the nature of the 

 crystalline schists and a study of the area lying immediately to the 

 east of that. mapped by Logan made it evident that Logan's conclusions 

 must be in part revised. 



The Fundamental Gneiss, in the first place, is found to be a great 

 body of uniform, fine-grained, foliated granite, showing under the 

 microscope excellent protoclastic structure. It is clearly an igneous 

 intrusion in which foliation has been developed by movement under 

 pressure. The suggestion of stratification which it presents owing 

 to its foliation, is enhanced by the presence in it of occasional lenticular 

 bands of dark amphibolite. These, of course, lie parallel to the 

 foliation, that is, to the direction of movement in the rock, but are 

 evidently inclusions of the overlying rock which was intruded by the 

 granite. The thickness of this fundamental gneiss (5,000 feet), 

 considered even by I^ogan to be wholly conjectural, must therefore be 

 deducted from the total thickness given in the above section. 



The anorthosite, when traced to the east, is also proved beyond 

 doubt to consist of a great body of igneous rock having a pronounced 

 foliation, especially near its margin, and which cuts off the limestone 

 bands where it meets them. Its thickness (10,000 feet), which was 

 also considered by Logan to be wholly conjectural, must also be 

 deducted. 



It is furthermore certain, as the result of recent work, that the 

 gneisses associated with the limestones are in part of igneous origin 



I "The Chemistry of Metamorphic Rocks," Chemical Geological Essays, 

 Boston, 1875. 



