36 S F. H. Lahee — Metamorphism and Geological Structure. 



amount of metamorphism varies with the depth, being greater 

 at the bottom (of bore-holes) than near the surface." Cobb 

 drew this conclusion from his study, in 1887, of a set of cores 

 obtained from two bore-holes in Portsmouth, R. I. (near Loc. 

 34, H : 9, fig. 21). We have made a thorough re-examination 

 of the same cores with the following results : 



Hole 1 



Hole 2 



Actual 



Stage of 



Actual 



Stage of 



depth 



Rock metam. 



depth 



Eock metam. 









13' 



1" 



Coarse sandstone 



A 









36' 



t 



Medium " 



C 









92' 



s" 



a a 



A 









102' 



1" 



u a 



A 









114' 



2" 



Coarse " 



A 









129' 



3" 



Fine, dark shale 



A 









149' 



3" 



Medium sandstone 



B 



286' 11" 



Fine, dark shale 



c 



194' 



Hit 



1 



Banded shale 



A 



109' 10" 



a a a 



A 

















233' 



2" 



Medium sandstone 



A 



244' 



it a a 



B 

















284' 



7" 



n tt 



B 









325' 



11" 



Coarse shale 



A 









334' 



4" 



Medium sandstone 



A 



336' 10" 



Fine sandstone 



B 

















380' 



3" 



Coarse sandstone 



A 



484' 4" 



Coarse shale 



C 











567' 4" 



u a 



C 











In Hole 1, if it were not for the first mentioned shale, the 

 metamorphism would show a regular increase downward ; but 

 this specimen proves that high metamorphism is not necessarily 

 limited to great depth in the Basin. 



In Hole 2 there is practically no alteration from top to bot- 

 tom. An exception occurs at 36' 7", where the highest meta- 

 morphism in the boring is recorded. 



Although the data furnished by these specimens are scanty, 

 nevertheless the facts which they set forth are not to be dis- 

 regarded. They indicate (1) that, at least as low as 400 feet, 

 depth has no obvious effect, upon the intensity of the meta- 

 morphism, and (2) that a high degree of alteration may occur 

 near the surface and at considerable depths alike. 



(2) According to the interpretation of the structural geology, 

 strata in the Wiekford district are at about the same horizon 

 as those of the Bonnet ; yet the metamorphism is more intense 

 in the latter region. The rocks of Dutch Island (Loc. 14, 

 C-D : 13, fig. 21), Beaver Head (Loc, 20, C-D : 14), and west- 



