258 STRATIGRAPHICAL GEOLOGY. 



respecting the nature of the events transpiring in the Labrador period, — 

 given in the chapter upon the physical history of the strata, — pertain to 

 a later epoch. The Pemigewasset granites have no connection with the 

 labradorites. The theory of a connection between them was based partly 

 upon the nature of the rock underlying the Waterville labradorite. Our 

 first account of it was correct, in substance, that it was gneiss. Subse- 

 quently it was thought to consist of the "trachytic" or Albany granite; 

 and the erroneous estimate of the Pemigewasset series based upon it. 

 Our further study of the specimens proved the rock to be a part of the 

 porphyritic gneiss area (p. 156); and the reexamination of the ledges con- 

 firmed the return to nearly the original supposition. Then a reconsidera- 

 tion of the mutual relations of the porj^hyries and labradorites elsewhere 

 has not yet given any decided proof of their intimate association, nor of 

 their superposition upon the Albany granite. We are therefore led to be- 

 lieve that the labradorites alone represent the Labrador system, and, as 

 thus limited, it has been described (pp. 209-221). There are seven small 

 areas of it, cut by a sort of "sienite" containing triclinic feldspars, and 

 therefore supposed to close the period. The reduction of the Labrador 

 White Mountain areas will render it necessary to abbreviate also those 

 described upon page 11, Chapter I, The reality of the system is not af- 

 fected by the removal from it of these various porphyries and granites. 

 Their elimination makes the correspondence perfect between the New 

 Hampshire and Canadian areas, thus establishing more firmly the exist- 

 ence of the series. 



Some difference of opinion may exist among geologists as to the rela- 

 tions between the Montalban and Labrador systems. We have not found 

 reason to change the conclusions derived from the section along the Mt. 

 Washington river (PI. VIII, Fig. 20). The labradorite rocks, with a very 

 moderate dip, rest unconformably upon the greatly upturned edges of the 

 Montalban schists, as if there had been large upheavals at the close of 

 the Montalban period, and comparatively little disturbance since. I un- 

 derstand that Dr. Eugene A. Smith, state geologist of Alabama, finds a 

 similar state of things in his field of labor — labradorites resting upon 

 Montalban schists. The facts as interpreted arc of great consequence, 

 since they fix the geological horizon of the whole Atlantic system, while 

 considerations of a stratigraphical character confirm this impression. 



