2 THE JOURNAL OF GEOLOGY. ' 



in.terpretations of the facts. Only within recent years has 

 detailed and independent work been undertaken in widely sepa- 

 rated parts of this vast area, and as yet no sufficient data is at 

 hand for structural, or even for petrographical correlation 

 throughout the whole. 



Complete geological maps, showing the structural relations 

 and chronological sequence of all the crystalline formations, are 

 undoubtedly what must be looked forward to as the ultimate aim 

 of work within this region, but the most sanguine will surely 

 admit that we are at present a long way from any such reality. 

 Meanwhile, in the absence of paleontological evidence, the study 

 of the rocks from the point of view of genesis and the establish- 

 ment of petrographic correlations will do much toward furnish- 

 ing the positive basis of knowledge upon which final solution of 

 complex structure must rest. 



Some of the notions regarding petrographic sequence and 

 the origin of foliation, enforced by masters of geology high in 

 authority, have obscured rather than advanced the problems pre- 

 sented by the crystalline rocks in eastern North America. Not 

 only have we been taught that the mineralogical and structural 

 characters of these rocks are safe indices of their superposition 

 and relative age, but the interpretation of all parallel structures 

 as proofs of sedimentation has led to the conclusion that igneous 

 rocks are rare, if not altogether absent, 'in these oldest and gen- 

 erally foliated formations of the earth's crust. Now, however, 

 better conceptions are beginning to prevail. No longer do we 

 regard the petrographic character of a crystalline rock as any 

 criterion of its age, while modern methods have enabled us to 

 identify the abundant igneous rocks of ancient times in spite 

 of the misleading structures imparted to them by secondary 

 causes. 



Object of this paper. — The present writer has had frequent 

 occasion to insist on the presence of such disguised igneous 

 masses in the oldest geological formations, and to dwell upon 

 the methods by which their origin may be established. In the 

 present paper it is his object to show that not only igneous, but 



