216 B. K. Emerson— Northfieldite. 



The norm calculated from the analyses is 



Quartz 



83*70 



Orthoclase 



T-78 



Albite 



4-19 



Anortkite 



1-67 



Hypersthene 



1-89 



Ilmenite 



0-30 



According to the C. I. P. W. quantitive classification the 

 northfieldite belongs in class 1 and order 2, but with so large 

 an excess of silica that it almost goes into the unoccupied 

 order 1. It may be expressed : 1. (1)2. 1-2. 3. It is a cardif- 

 fose. The difference between analyses 3 and 4 as compared 

 with 1 and 2 depends largely on the presence of actinolite 

 probably due to the introduction of limestone from the cover, 

 and if the analyses be recalculated with the actinolite omitted, 

 4 agrees closely and 3 approximately with the first two 

 analyses. 



In the region of transition from the basic to the acid border 

 beds on the north line of Pelham and two miles north of the type 

 locality on Mt. Orient both border types, the coarse and the fine, 

 are present and present in their normal relations to the subjacent 

 granite gneiss. The northfieldite is present in an area nearly 

 a mile square, as a white fine-grained sugary often friable mass 

 with a little biotite or actinolite. The banking of the gneiss 

 can be seen dipping under it on either side with very low 

 angles. Long bands of a black, rather coarse diorite some- 

 times banded, sometimes massive, rest upon the northfieldite 

 which dips beneath them from either side. These bands are 

 not very thick and may be assumed to be the remnants spared 

 by erosion of a continuous layer separating the northfieldite 

 from the overlying sedimentary schist into which the great 

 batholite penetrated. It may well have capped theMt. Orient 

 area to the south, but soon disappeared on the north as the 

 northfieldite is there found in contact with the overlying schists 

 without any diorite band between. Indeed, in Crag Mountain 

 a band of highly muscovitic rock (pegmatite schist) takes its 

 place. 



A quartzite like the Crag Mountain rock appears in the 

 lower stage of the Freiberg gneiss, which probably cuts Car- 

 boniferous beds, in a thick lens or stock form, sometimes banded 

 and stretched like the adjacent gneiss and contains muscovite 

 and sometimes tourmaline, and rutile. It is thought to be a 

 most acid after-intrusion of the gneiss.* 



A similar border bed occurs at Adriaans Kop in So. Africa 

 with 97-43 of quartzf and in Eskdale, England, with 96*43 



* C. Gabert : Die Gneisse des Erzgebirge, tmd ibre Coiitact-wirkungen. 

 Zeit. d. Deutscben Geologiscben Gesell, vol. lix. p. 322, 1907. 

 flddings, I. P., Igneous Kocks, ii, p. 29, 1913. 



