146 Barrell — Growth of Knowledge of Earth Structure. 



fusion. The baseless and speculative character of the 

 use of minerals as an index of age and of Hunt's inter- 

 pretation of New England geology in general was shown 

 by Dana in 1872 (3, 91). The following year Dana 

 pointed out clearly that igneous eruptions in general 

 have been derived from a deep-seated source and did not 

 come from the aqueo-igneous fusion of sediments. As to 

 gradations between true igneous rocks and fused and 

 displaced sediments he makes the following statements 

 (6, 114, 1873) : 



"Again, the plastic rock-material that may be derived from 

 the fusion or semifusion of the super crust, (that is, of rocks 

 originally of sedimentary origin, ) gives rise to ' ' igneous ' ' rocks 

 often not distinguishable from other igneous rocks, when it is 

 ejected through fissures far from its place of origin ; while crys- 

 talline rocks are simply metamorphic if they remain in their 

 original relations to the associated rocks, or nearly so. 



Between these latter igneous rocks and the metamorphic there 

 may be indefinite gradations, as claimed by Hunt. But if our 

 reasonings are right, the great part of igneous rocks can be 

 proved to have had no such supercrust origin. The argument 

 from the presence of moisture or of hydrous minerals in such 

 rocks in favor of their origin from the fusion of sediments has 

 been shown to be invalid. ' ' 



The injected marginal rocks and the post-intrusive 

 metamorphism of most of the New England granites has, 

 however, obscured more or less their real igneous nature 

 so that the gradation from metamorphic sediments 

 through igneous gneisses to granites could be read in 

 either direction. These features misled Dana who 

 accepted the prevailing idea of the general metamorphic 

 origin of granite. Dana makes the following statement 

 (6, 164, 1873) : 



"But Hunt is right in holding that in general granite and 

 syenite (the quartz-bearing syenite) are undoubtedly meta- 

 morphic rocks where not vein-formations, as I know from the 

 study of many examples of them in New England; and the 

 veins are results of infiltration through heated moisture from 

 the rocks adjoining some part of the opened fissures they fin." 



Granite, although regarded at this time as the extreme 

 of the metamorphic series and originating from sedi- 

 ments, was looked upon as typically Archean in age, 

 though in some cases younger. Such a doctrine per- 

 mitted such extreme misinterpretations as that of 



