426 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



since the time of metamorphism tliey have been subjected to stress, as is 

 clearly shown by the optically strained condition of the secondary quartz 

 g-rains and the bending and twisting of the micas. 



From these facts we may reasonably infer that the general metamor- 

 phism of both series was accomplished after the deposition of the upper 

 series and before the latter was folded. Reconstruction so complete as 

 that shown by the upper series is not believed to take place except at con- 

 siderable depths below the surface, and hence the part of the upper series 

 now visible must then have been deeply covered by overlying rocks, which 

 were afterwards entirely swept away before the deposition of the Cambrian. 

 In the earth movements which folded this mass of material and brought it 

 up within the reach of denuding agents, we may recognize the causes which 

 have strained and broken the secondary minerals of both series alike. 



SECTIOX IX. THE INTBITSIYT^S. 



The Algonkian formations of the Felch Mountain area have been cut 

 by later intx-usives, among which both acid and basic rocks are represented. 

 The latter have also been recognized in the Archean, in which, indeed, the 

 freshest and least-altered occurrences have been found. 



The acid rocks consist of fine- to medium-g-rained pink granites, 

 occurring in narrow dikes. A number of these dikes have been found in 

 the Sturgeon formation, both in the area of line exposure on the south side 

 of sec. 35, T. 42 N., R. 30 W., and also in sees. 34 and 35, T. 42 N., R 29 W. 



Two granite dikes are also known in the highest member of the lower 

 series, but none have been detected in the Randville or ]\Iansfield forma- 

 tions. One of these occurs on Felch Mountain, the other, a very coarse 

 pegmatite, is found cutting the Grroveland formation in the southern part of 

 sec. 33, T. 42 N., R. 28 W. 



Basic dikes and intrusive sheets are found in many localities. Some 

 are highly schistose and greatly altered, others are massive and but little 

 changed. They probably belong to many eras of eruption. The least 

 altered are diabases, in one occui-rence of which, from the Archean, the 

 ausrites are almost intact. 



