130 (>'. S. Rogers— Original Gneissoid Structure. 



promptly begun to cool and harden while resting- quietly, as 

 otherwise the two magmas might combine to form a third and 

 homogeneous one. Mr. Harker's alternative view is that the 

 mass was intruded as a unit, ahead}' heterogeneous, the two 

 different magmas having been partly mixed before intrusion. 

 Whichever be the correct theory, it is evident that in the Cort- 

 landt Series the simple norite magma was very small in com- 

 parison with the more complex norite magmas, since the former 

 always appears as included bands in the others, while these 

 latter cover extensive areas. 



It is interesting to note that here, as in most of the other 

 examples of this structure, there have been distinct changes in 

 our conception of its significance. The old school of Ameri- 

 can geologists, of which Professor Dana was the last great 

 disciple, were very prone to consider as worked-over sedi- 

 ments svhat are now called true igneous masses ; and he there- 

 fore even adduced this structure to prove the sedimentary 

 origin of the rocks. Dr. Williams, on the other hand, was one 

 of the early exponents of the school which has laid great stress 

 on metamorphie action in igneous rocks, and he accordingly 

 passed it over without concern, as being merely an evidence 

 of regional metamorphism. To-day we are passing — or perhaps 

 have passed — through yet another change; Vogt, Hogbom, 

 Harker, Adams, Pirsson, Kemp and others are now, in the 

 light of our present greater knowledge, reading into igneous 

 action many attributes, for the manifestation of which an 

 entirely different origin had hitherto been postulated. The for- 

 mation of this original gneissoid structure is a case in point. 

 Up to the present it has not "been recognized in many localities ; 

 should it be found to be more common than is now thought, 

 however, it may prove illuminating (as Sir Archibald Geikie 

 suggests) in connection with some of the puzzling structures 

 of the ancient and obscure igneous gneisses. 



Columbia University, New York. 



